Clip Peak Calling

What the source does

• FilesystemNone observedNo reads or writes in the source
• NetworkNone observedNo outbound calls in the source
• ShellNone observedNo commands run in the source
• DependenciesPinnedNothing floats
• SecretsNone foundNo keys in source

Five audit layers

Gates applied: no behavioural pass.

Audit 0.4.1 · source sha c8c1fc3a2b13 · 19 Oct 2026Report an issue or request a re-scan

• PublisherGPTomics GitHub organizationOwner of the repository this skill is read from
• Source folderGPTomics/bioSkills/clip-seq/clip-peak-callingThe exact folder we audited
• LicenceMITAs the repository states it
• Commit pinned for this verdictc8c1fc3a2b13A new audit runs when the source changes
• Repository statusArchivedLast push 65 days ago
• Sandboxed runNot runSkills are read, not executed; layer L4 is skipped

Its description: Call protein-RNA binding sites from CLIP-seq BAM with CLIPper, PureCLIP, Skipper, Piranha, omniCLIP, CTK, CLAM, or Paraclu. Use when choosing between coverage-based, HMM-based, beta-binomial window-based, and…

---
name: bio-clip-seq-clip-peak-calling
description: Call protein-RNA binding sites from CLIP-seq BAM with CLIPper, PureCLIP, Skipper, Piranha, omniCLIP, CTK, CLAM, or Paraclu. Use when choosing between coverage-based, HMM-based, beta-binomial window-based, and crosslink-site-based peak callers; applying ENCODE eCLIP thresholds (log2 IP/SMInput >= 3, -log10 p >= 3); deciding when SMInput is mandatory; or reconciling peak-set discordance between callers for the same RBP.
tool_type: cli
primary_tool: CLIPper
---

## Version Compatibility

Reference examples tested with: CLIPper 2.0+, PureCLIP 1.3.1+, Piranha 1.2.1+, omniCLIP 0.2.0+, CTK 1.1.4+, CLAM 1.2+, Paraclu 9+, Skipper (commit 2023.05+), MACS3 3.0+, bedtools 2.31+, samtools 1.19+, idr 2.0.4+.

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

If code throws unexpected errors, introspect the installed binary and adapt the example to match the actual CLI rather than retrying. PureCLIP 2.x changed several flag names; Skipper is distributed as a Snakemake workflow with frequently-evolving paths.

# CLIP-seq Peak Calling

**"Call protein-RNA binding sites from my deduplicated CLIP BAM"** -> Identify regions where read pile-up (and, for iCLIP/eCLIP, single-nucleotide truncations) exceed background from a size-matched input (SMInput) control. The choice of peak caller depends on (a) the CLIP variant (HITS-CLIP, iCLIP, eCLIP, PAR-CLIP), (b) whether SMInput is available, (c) the RBP binding mode (narrow motif vs broad zones vs repeat-binding), and (d) the goal (publication-comparable ENCODE peaks vs single-nucleotide crosslink sites vs high-recall site lists).

- CLI (ENCODE eCLIP canonical): `clipper -b dedup.bam -s hg38 -o peaks.bed --save-pickle --FDR-alpha 0.05 --superlocal` then `eclip-norm peaks.bed -i sminput.bam` for log2 fold-change against SMInput (`--FDR-alpha` is the CLIPper flag for the FDR cutoff; older docs sometimes shorten to `--FDR`)
- CLI (single-nucleotide crosslink sites, iCLIP/eCLIP): `pureclip -i dedup.bam -bai dedup.bam.bai -g genome.fa -ibam sminput.bam -ibai sminput.bam.bai -o sites.bed -or regions.bed -nt 8 -dm 8`
- CLI (high-recall, beta-binomial windowed; needs SMInput): `Skipper` Snakemake workflow with config matching cell type and SMInput BAM
- CLI (no SMInput, no truncation): `Piranha -b 50 -p 0.01 -d ZeroTruncatedNegativeBinomial -s -o peaks.bed dedup.bam` (Piranha takes the BAM as a positional argument, not via `-s`)
- CLI (CIMS/CITS single-nt from CTK): `tag2cluster.pl dedup.bed cluster.bed --multi-tag-method coverage`; then `bedExtractCIMS.pl cluster.bed cims.bed`
- CLI (multi-mapper rescue for repeat-binding RBPs): `CLAM peakcaller -i unique.bam multimap.bam -o clam_out_dir/ --gtf gencode.gtf` (CLAM peakcaller writes peaks into an OUTPUT DIRECTORY, not a single file)

The ENCODE eCLIP gold standard for "stringent" peaks is: log2(IP / SMInput) >= 3 AND -log10(p-value) >= 3. "Lenient" peaks use log2 >= 1 AND -log10 >= 2. Both filters operate on CLIPper peak output normalized against SMInput. Without SMInput, neither stringency level can be reproduced - the algorithm has no background term.

## Algorithmic Taxonomy

| Caller | Model | Resolution | SMInput required | Strength | Fails when |
|--------|-------|------------|------------------|----------|------------|
| CLIPper (Yeo) | Poisson with 500 bp local lambda, cubic-spline interpolation | Peak (10-500 nt) | Not for calling; required for ENCODE normalization | ENCODE eCLIP standard; reproducible against published ENCODE peak sets | Assumes most reads not from binding; fails when IP signal dominates a gene; sensitive to highly expressed transcripts |
| PureCLIP (Krakau 2017) | Non-homogeneous HMM jointly modeling enrichment + truncation + sequence biases | Single-nucleotide CL + broad region | Optional via -ibam; recommended | Single-nt resolution; only caller that explicitly models the iCLIP truncation pattern | Misses broad binding zones; very focal (low recall on broad-binding RBPs in the Boyle 2023 benchmark) |
| Skipper (Boyle 2023) | GC-stratified beta-binomial, 100 bp feature-respecting windows | Window (~100 nt) | Mandatory | 210-320% more sites than CLIPper; 8x faster; properly normalizes against input | Loses single-nt resolution; relatively new (2023); fewer published peak sets to compare |
| Piranha (Smith lab) | Zero-truncated negative binomial regression with optional covariates | Bin (50-200 nt) | Optional; pass as covariate | Mature, widely cited, handles count overdispersion | Biased toward high-expression transcripts; convergence fails with large covariate values (use `-l` log-space) |
| omniCLIP (Drewe-Boss 2018) | Non-homogeneous HMM with Dirichlet-multinomial of variants | Peak | Required | Models replicate variance; integrates background; can call peaks on any CLIP variant | Slow on deep libraries; blind to mitochondrial transcripts (misses chrM windows for FASTKD2) |
| CTK CIMS/CITS (Shah 2017) | Empirical FDR on crosslink-induced mutations or truncations | Single-nucleotide | No (uses background mutation rate from non-bound transcripts) | Single-nt resolution; works on HITS-CLIP deletions, PAR-CLIP T->C, iCLIP truncations | Empirical FDR less principled than HMM/beta-binomial; perl-based pipeline harder to integrate |
| CLAM peakcaller (Zhang & Xing 2017) | EM-assigned multi-mapper count + Piranha-like negative binomial | Peak | Optional | Only solution for repeat-binding RBPs; 10-30% additional sites in repeats | Inherits Piranha limitations on coverage-based stats; slower than Piranha |
| Paraclu (Frith) | Parametric clustering with min-density and max-density thresholds | Variable | No | Simple, parameter-tunable, works on bedgraph | Heuristic; no statistical significance; cluster boundaries sensitive to thresholds |
| PIPE-CLIP | Online CLIP pipeline with

Excerpt, first 6,000 characters of SKILL.md, verbatim from the audited commit. It is text from someone else’s repository, quoted so you can judge it before your agent reads it.

git clone https://github.com/algaebrown/skipper && cd skipper && conda env create -f environment.yaml
git clone https://github.com/chaolinzhanglab/ctk
pip install CLAM

git clone https://github.com/GPTomics/bioSkills
git -C bioSkills checkout c8c1fc3a2b13

Works in

1. 2026-10-19
   ✓SAFE B 89 / 100

   first audit · source c8c1fc3a2b13

Questions about Clip Peak Calling

• github.com/GPTomics/bioSkills/tree/main/clip-seq/clip-peak-calli — the audited source at commit c8c1fc3a2b13 and every finding aboveObserved · 19 Oct 2026

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