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name: bio-clip-seq-crosslink-site-detection description: Detect single-nucleotide crosslink (CL) sites in CLIP-seq data using truncation patterns (iCLIP/eCLIP CITS), crosslink-induced mutations (HITS-CLIP CIMS deletions, PAR-CLIP T-to-C), or HMM/kernel-density methods (PureCLIP, PARalyzer, CTK). Use when single-nucleotide resolution is required for motif registration (mCross), allele-specific binding (BEAPR), variant-effect prediction, or comparing crosslink chemistry across CLIP variants. origin: openai4s category: bioskills/clip-seq metadata: tool_type: cli primary_tool: PureCLIP third_party: name: GPTomics/bioSkills repository: https://github.com/GPTomics/bioSkills commit: d91ed3d563019e649dc854c56ccd62551359488a license: MIT

Version Compatibility

Reference examples tested with: PureCLIP 1.3.1+, CTK 1.1.4+, PARalyzer 1.5+, wavClusteR 2.34+, pyCRAC 1.5+, samtools 1.19+, bedtools 2.31+, pysam 0.22+, R 4.3+.

Before using code patterns, verify installed versions match. If versions differ: - CLI: <tool> --version then <tool> --help to confirm flags - Python: pip show <package> then help(module.function) to check signatures - R: packageVersion('<pkg>') then ?function_name to verify parameters

If code throws unexpected errors, introspect the installed tool and adapt the example to match the actual CLI rather than retrying.

CLIP-seq Crosslink-Site Detection

"Detect single-nucleotide crosslink sites in my CLIP data" -> Identify the exact base where the protein-RNA UV adduct caused the reverse transcriptase to stop (truncation, in iCLIP/eCLIP), to read through with a mutation (deletion in HITS-CLIP, T->C in PAR-CLIP), or to leave a multi-base signature (PARalyzer kernel density for PAR-CLIP). Single-nucleotide resolution is the foundation of motif registration (mCross), allele-specific binding (BEAPR/ASPRIN), variant-effect prediction, and the most rigorous comparisons across CLIP variants. The detection chemistry differs by CLIP type: iCLIP/eCLIP read 5' end is one nucleotide downstream of the crosslink (CITS); PAR-CLIP reads contain T->C transitions at the crosslink (CIMS substitution); HITS-CLIP reads contain deletions at the crosslink (CIMS deletion).

Single-nt CL sites are the input to mCross motif registration (see clip-seq/clip-motif-analysis), to BEAPR/ASPRIN allele-specific binding analyses, and to RBPNet/DeepRiPe deep-learning models (see clip-seq/clip-deep-learning). They are NOT a replacement for broad peak calls; the two outputs are complementary. A common downstream error is passing a CLIPper peak BED to mCross instead of a PureCLIP single-nt BED - mCross requires the single-nt resolution to register motif position relative to the crosslink offset.

The detection method must match the underlying chemistry of how the reverse transcriptase encountered the protein-RNA adduct:

CLIP variant RT behavior at CL Detection signature Sensitivity (CL captured per read) Sequence bias
iCLIP / iCLIP2 / iCLIP3 / eCLIP / irCLIP / FLASH Truncates at adduct 5' end of cDNA = CL site - 1 ~80% of reads truncate; the minority read through Strong U bias at CL sites
HITS-CLIP Reads through with deletion (~8-20% of tags) Single-base deletion in read ~8-20% of tags contain a deletion at the CL site U bias plus sequence-specific deletion rate
PAR-CLIP (4SU) Reads through with T->C (20-50% of T positions in crosslinked reads) T->C transition (or G->A on reverse strand) Per-T rate 20-50%; per-read 1-5 conversions Restricted to T positions; depends on 4SU incorporation rate
PAR-CLIP (6SG) Reads through with G->A G->A transition Lower rate than 4SU T->C Restricted to G positions
miCLIP / miCLIP2 (m6A) Primarily truncation at the m6A site (RT stops at antibody-trapped m6A); secondary C->T transition observed in a subset of reads at m6A 5' end (CL -1); C->T rate is a feature used by m6Aboost ML in addition to truncation, not a stand-alone primary signal Mixed; m6Aboost ML integrates truncation + sequence context to score DRACH motif context required
STAMP C->T editing on target (not crosslink) C->T editing in mRNA reads NA (editing-based; not crosslink-based) N/A
TRIBE A->I editing on target A->G in cDNA (I read as G) NA Adjacent to ADAR consensus

Algorithmic Taxonomy

Tool CLIP variant Detection signal Statistical model Output resolution Strength Fails when
PureCLIP (Krakau 2017) iCLIP/eCLIP/PAR-CLIP Truncation + enrichment + CL motif Non-homogeneous HMM Single-nucleotide The most comprehensive HMM model Lowest F1 among callers benchmarked in Boyle 2023 on broad-binding RBPs (highly focal)
CTK CITS (Shah 2017) iCLIP/eCLIP Truncation only Empirical FDR vs background Single-nucleotide Simple, well-validated for iCLIP Less granular than PureCLIP; no HMM smoothing
CTK CIMS deletion (Shah 2017) HITS-CLIP Single-base deletions Empirical FDR Single-nucleotide Standard for HITS-CLIP Requires deletion-tolerant aligner (BWA -e)
CTK CIMS T->C (Shah 2017) PAR-CLIP T->C substitutions Empirical FDR Single-nucleotide Alternative to PARalyzer Less popular than PARalyzer
PARalyzer (Corcoran 2011) PAR-CLIP T->C transitions, kernel density Kernel density estimation Cluster (10-50 nt) Field standard for PAR-CLIP Cluster-level, not single-nt; needs careful parameter tuning
wavClusteR (Comoglio 2015) PAR-CLIP T->C with wavelet smoothing Wavelet transform + clustering Cluster Robust to sequencing depth variability Less single-nt; legacy R package
pyCRAC / kPLogo CRAC (yeast) Read truncation + deletion Empirical Single-nucleotide Original CLIP analytics tool Yeast-focused; perl/python legacy
Piranha bins Any Coverage in bins ZTNB Bin-level (50-200 nt) Not crosslink-specific Bin width too coarse for single-nt
HOMER tag2pos Any 5' end position None Read-end positions only Quick truncation site dump No statistical filtering
omniCLIP Any Coverage + variant pattern Dirichlet-multinomial HMM Region Not focal Too broad for single-nt
iCount (paths) iCLIP Crosslink site cluster Empirical Cluster Used in nf-core/clipseq Less single-nt than PureCLIP

Methodology evolves; PureCLIP 2.0 and CTK 1.2 have minor flag changes; PARalyzer parameters need careful tuning per-RBP. Cross-validate single-nt sites with at least two tools when high-confidence reporting is required.

Critical Choice: Truncation vs Mutation vs HMM

Three orthogonal approaches:

Truncation-based (CITS, PureCLIP) -- Find positions enriched in cDNA 5' ends. The RT enzyme stops at the adduct; the 5' end of the read maps to CL site - 1. Used for iCLIP/eCLIP. Pro: high yield (~80% of reads truncate). Con: U bias of crosslinking inflates U positions.

Mutation-based (CIMS for deletions/substitutions; PARalyzer for T->C) -- Find positions with crosslink-induced mutations. RT reads through the adduct with high mutation rate at the CL position. Used for HITS-CLIP (deletions, ~8-20%) and PAR-CLIP (T->C, 20-50%). Pro: less U bias; PAR-CLIP signal is restricted to T positions. Con: lower yield (only mutated reads count).

HMM-based (PureCLIP, omniCLIP) -- Joint model of enrichment + CL signature + sequence context. State the most-likely posterior probability of CL state per nucleotide. Used across CLIP variants. Pro: integrates multiple signals; explicit input normalization. Con: F1 ~0.2 on bulk RBPs (very focal); single-nt resolution at the cost of breadth.

Goal Method Tool
Single-nt CL map for motif registration (mCross) HMM PureCLIP
Truncation-based ENCODE-compatible iCLIP Truncation CTK CITS
PAR-CLIP T->C signal Mutation PARalyzer (cluster) or CTK CIMS sub T->C (single-nt)
HITS-CLIP deletion signal Mutation CTK CIMS deletion
Cross-CLIP-variant comparable single-nt HMM PureCLIP (all variants)
Allele-specific CLIP (BEAPR) Truncation + ASB PureCLIP CL sites + WASP-filtered BAM
Variant effect (computational from CL) HMM PureCLIP + downstream DeepRiPe (see clip-deep-learning)
Yeast / CRAC Truncation + deletion pyCRAC

Per-Tool Failure Modes

PureCLIP -- HMM convergence on sparse coverage

Trigger: RBP with low IP enrichment; SMInput depth uneven; PureCLIP run on full genome without -iv interval.

Mechanism: PureCLIP HMM convergence is sensitive to coverage depth. On regions with < 5 reads/100 bp the HMM defaults to background state; if half the genome is sparse, the state distributions become bimodal and the entire run takes > 24 h or runs out of memory.

Symptom: PureCLIP "Convergence not reached" warning; or output has 0 CL sites; or runtime > 24 h on standard server.

Fix: Restrict analysis to expressed transcripts: -iv expressed_tx.bed. Or filter the BAM to high-coverage regions first. Test on chr22 first to verify parameters before genome-wide run.

PureCLIP -- Misses broad binding zones

Trigger: RBP with broad binding (PUM2 3' UTRs, SR proteins exonic enhancers); using PureCLIP exclusively.

Mechanism: PureCLIP HMM emits the CL state at very high stringency. The Boyle 2023 Skipper benchmark reports PureCLIP as the most focal caller with low recall on broad-binding RBPs.

Symptom: PureCLIP CL site count is 100x lower than CLIPper / Skipper peak count.

Fix: Use PureCLIP for single-nt CL maps AND CLIPper/Skipper for broad-zone peak calls. They are complementary, not substitutes. PureCLIP -or regions output is broader than -o sites but still focal.

CITS truncation -- Misapplied to PAR-CLIP

Trigger: CTK CITS run on PAR-CLIP data.

Mechanism: PAR-CLIP RT reads through the adduct (T->C mutation), not truncates. The 5' end of PAR-CLIP reads is at the fragment 5' end, not the CL site.

Symptom: CITS returns few sites; the sites that ARE returned do not align with the T->C mutation positions.

Fix: For PAR-CLIP use PARalyzer or CTK CIMS substitution mode (-type sub -nuc t -mut c). CITS is iCLIP/eCLIP only.

CIMS deletion -- Misapplied to iCLIP

Trigger: CTK CIMS run with -type del on iCLIP data.

Mechanism: iCLIP RT truncates, doesn't delete. The deletion rate is < 1% in iCLIP (background level), so CIMS finds nothing significant.

Symptom: CIMS output BED has < 100 sites genome-wide.

Fix: Use CTK CITS for iCLIP (truncation-based) or PureCLIP. CIMS deletion is HITS-CLIP / older PAR-CLIP only.

PARalyzer -- Parameter sensitivity

Trigger: PARalyzer default parameters on a new RBP; cluster count unexpected.

Mechanism: PARalyzer's kernel-density approach has 5+ parameters: minimum read depth, minimum mutation read count, mutation rate threshold, bandwidth, kernel type. Different combinations produce 5-100x different cluster counts.

Symptom: PARalyzer cluster count 5-100x off from literature reports for the same RBP.

Fix: Use the PARalyzer default parameters (Corcoran 2011), tuned for that RBP class. Validate on a known-binding region with a reference dataset before publication.

Aligner deletion-tolerance for HITS-CLIP

Trigger: HITS-CLIP aligned with STAR or bowtie2 in standard mode; CIMS finds few deletion sites.

Mechanism: Standard STAR/bowtie2 are aggressive about handling deletions; CIMS expects the deletions to be visible as CIGAR D operations in BAM. STAR/bowtie2 may soft-clip or fail to align deletion-bearing reads.

Symptom: Few CIGAR D operations in BAM (samtools view dedup.bam | awk '$6 ~ /D/' | wc -l); CIMS deletion site count low.

Fix: Re-align HITS-CLIP with BWA-aln (Zhang lab / CTK convention) or STAR --scoreDelOpen -1 --scoreDelBase -1 --scoreInsOpen -1 --scoreInsBase -1 to permit deletions. Or use Novoalign which is more deletion-tolerant.

PAR-CLIP T->C mismatch ceiling

Trigger: PAR-CLIP aligned with STAR --outFilterMismatchNoverReadLmax 0.04; T->C reads discarded.

Mechanism: PAR-CLIP per-T conversion rate 20-50% means a 30 nt read with 8 Ts may have 4 T->C events = 13% mismatch rate, exceeding the 4% ceiling.

Symptom: 40-70% read loss at alignment for PAR-CLIP; downstream T->C site detection finds nothing.

Fix: Raise to 0.07 for PAR-CLIP only (see clip-seq/clip-alignment).

Strand mis-assignment for truncation site

Trigger: Single-end CLIP; using read 5' end as CL site without strand consideration.

Mechanism: For plus-strand RNA, the cDNA 5' end is downstream of the CL position. For minus-strand RNA, it is upstream. Without strand-aware coordinate conversion, half of CL sites are mis-positioned by ~1 nt.

Symptom: mCross / motif analysis shows the motif positioned 1 nt off from expected.

Fix: Verify the BED output handles strand correctly. CTK CITS, PureCLIP, and most modern tools do this automatically; custom scripts must add if strand == '-': cl_position = read_end_5p + 1.

Decision Tree by Use Case

Scenario Tool Parameters
Single-nt CL map for mCross motif registration PureCLIP -i dedup.bam -ibam sminput.bam -dm 8
iCLIP/eCLIP CITS truncation sites CTK CITS tag2cluster.pl -cs5 5 -m 1
HITS-CLIP deletion CIMS CTK CIMS CIMS.pl -big -c -p 0.01
PAR-CLIP T->C clusters PARalyzer Per-RBP params tuned from Corcoran 2011 defaults
PAR-CLIP T->C single-nt CTK CIMS substitution CIMS.pl -type sub -nuc t -mut c -p 0.001
Yeast CRAC pyCRAC pyCRAC.py with HTP-tagged protein
Variant effect at CL sites PureCLIP + DeepRiPe See clip-seq/clip-deep-learning
Allele-specific binding WASP-filtered BAM + PureCLIP See clip-seq/clip-alignment for WASP
Cross-CLIP comparison PureCLIP (consistent across variants) Same parameters
m6A miCLIP2 sites miCLIP2-specific (m6Aboost) See clip-seq/m6a-clip
STAMP edit sites Not crosslink-based See clip-seq/stamp-antibody-free

Reconciliation: When Detection Methods Disagree

Pattern Likely cause Action
PureCLIP << CTK CITS site count PureCLIP HMM very focal; CITS empirical broader Both correct; report which threshold
PARalyzer clusters << CIMS T->C sites for PAR-CLIP PARalyzer is cluster-level; CIMS is single-nt Aggregate CIMS to clusters for comparison
PureCLIP CL sites at unexpected positions Strand mis-assignment OR aligner soft-clip Check BED strand column; verify --alignEndsType EndToEnd
HITS-CLIP CIMS empty Aligner not deletion-tolerant Re-align with BWA-aln or STAR with adjusted scoring
PAR-CLIP CIMS << expected Reads lost at mismatch ceiling Raise STAR --outFilterMismatchNoverReadLmax to 0.07
eCLIP CITS positions shifted by 1 nt Strand handling off-by-one Verify R2 5' end position handling
Multiple CITS sites within 5 nt RT stops near each other; same CL event Cluster within 10 nt window post-detection
Motif registers 5 nt off in mCross CL positions wrong by RT-stop offset Confirm tool reports "CL - 1" position consistently

Operational rule: For motif registration and ASB, run PureCLIP with SMInput. For ENCODE-comparable truncation-based output, also run CTK CITS. Cross-validate by checking that mCross motif PWM is the same when fed PureCLIP sites vs CTK CITS sites. If they diverge by > 2 nt in motif position, suspect strand or off-by-one issue.

Workflow: PureCLIP -> mCross -> ASB

# Step 1: PureCLIP single-nt sites
pureclip \
    -i sample.dedup.bam -bai sample.dedup.bam.bai \
    -g genome.fa \
    -ibam sminput.dedup.bam -ibai sminput.dedup.bam.bai \
    -o sample.crosslinks.bed \
    -or sample.regions.bed \
    -nt 8 -dm 8 -iv expressed_tx.bed

# Step 2: mCross motif registration
mCross -i sample.crosslinks.bed -g genome.fa -k 7 -n 5 -o mcross_out

# Step 3: Intersect with heterozygous SNPs for allele-specific binding
bedtools intersect -wa -wb -s -a sample.crosslinks.bed -b het_snps.vcf > cl_at_hets.bed

# Step 4: Test allele bias with BEAPR or ASPRIN
# (see allele-specific CLIP literature; not in this skill)

Common Errors

Error / symptom Cause Solution
PureCLIP "Convergence not reached" HMM didn't converge on sparse coverage Restrict to expressed transcripts with -iv expressed.bed
PureCLIP > 24h runtime Full genome on large library Test on chr22 first; add -iv filter
CTK CIMS empty for HITS-CLIP Aligner not deletion-tolerant Re-align with BWA-aln
CITS truncation sites not single-nt CIGAR S operations from soft-clip Verify --alignEndsType EndToEnd upstream
PAR-CLIP CIMS empty Reads lost at mismatch ceiling Raise STAR mismatch tolerance
Off-by-one motif position Strand handling differs Verify tool documents whether output is CL or CL-1
Single-nt motif lost in HOMER on PureCLIP sites Site BED too narrow; need flanking Extend bedtools slop -b 15 before motif analysis
mCross "no crosslinks found" Passed peak BED instead of CL BED Use single-nt site output
PureCLIP output looks like uniform distribution R2 5' end trimmed in preprocessing Re-preprocess; -g and --trim_front2 banned for CLIP
Strand-specific motif inverted BED column 6 wrong Verify strand encoding; PureCLIP preserves correctly

References