ChIP-seq Protocol Step by Step: Chromatin Immunoprecipitation Sequencing

LabProtocol.co Team·2026-03-22·8 min read
ChIP-seqepigeneticschromatinprotocolsnext-gen-sequencing

ChIP-seq Protocol Step by Step: Chromatin Immunoprecipitation Sequencing

Chromatin immunoprecipitation followed by sequencing (ChIP-seq) is the gold standard method for mapping protein-DNA interactions across the genome. Whether you are studying histone modifications (H3K4me3, H3K27ac, H3K27me3), transcription factor binding sites, or RNA polymerase II occupancy, ChIP-seq provides genome-wide binding profiles at near-nucleosome resolution.

This protocol covers the complete ChIP-seq workflow: crosslinking, chromatin preparation, sonication, immunoprecipitation, DNA purification, library preparation, and sequencing considerations.

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Materials and Reagents

Crosslinking and Lysis

  • 37% formaldehyde (methanol-free, Pierce #28906)
  • 2.5 M glycine (quenching solution)
  • Cell lysis buffer: 5 mM PIPES pH 8.0, 85 mM KCl, 0.5% NP-40, plus protease inhibitors
  • Nuclear lysis buffer: 50 mM Tris-HCl pH 8.0, 10 mM EDTA, 1% SDS, plus protease inhibitors
  • Protease inhibitor cocktail (Roche cOmplete Mini)

Immunoprecipitation

  • ChIP dilution buffer: 16.7 mM Tris-HCl pH 8.0, 167 mM NaCl, 1.1% Triton X-100, 1.2 mM EDTA, 0.01% SDS
  • ChIP-grade antibody against your target (2-5 ug per IP; e.g., H3K4me3 Abcam ab8580, H3K27ac Abcam ab4729, CTCF Millipore 07-729)
  • Protein A/G magnetic beads (Dynabeads)
  • Normal rabbit IgG (negative control)
  • Low-salt wash buffer: 20 mM Tris-HCl pH 8.0, 150 mM NaCl, 2 mM EDTA, 1% Triton X-100, 0.1% SDS
  • High-salt wash buffer: 20 mM Tris-HCl pH 8.0, 500 mM NaCl, 2 mM EDTA, 1% Triton X-100, 0.1% SDS
  • LiCl wash buffer: 10 mM Tris-HCl pH 8.0, 250 mM LiCl, 1 mM EDTA, 1% NP-40, 1% sodium deoxycholate
  • TE buffer: 10 mM Tris-HCl pH 8.0, 1 mM EDTA

Elution and Purification

  • Elution buffer: 1% SDS, 0.1 M NaHCO3
  • Proteinase K (20 mg/mL)
  • RNase A (10 mg/mL)
  • Phenol-chloroform-isoamyl alcohol (25:24:1) or PCR purification kit (Qiagen MinElute)
  • 5 M NaCl

Library Preparation and Sequencing

  • ChIP-seq library prep kit (e.g., NEBNext Ultra II DNA Library Prep Kit for Illumina, NEB E7645)
  • Illumina-compatible adapters and index primers
  • AMPure XP beads (Beckman Coulter) for size selection
  • Bioanalyzer or TapeStation for QC
  • Qubit fluorometer for quantification

Step-by-Step ChIP-seq Protocol

Step 1: Crosslinking

  1. Start with 1-2 x 10^7 cells per ChIP reaction (more for transcription factors, less for abundant histone marks).
  2. Add formaldehyde directly to the culture medium to a final concentration of 1%. Mix by gentle swirling.
  3. Incubate at room temperature for 10 minutes (for histone marks) or 10-15 minutes (for transcription factors).
  4. Quench crosslinking by adding glycine to a final concentration of 125 mM. Incubate 5 minutes at room temperature.
  5. Wash cells twice with ice-cold PBS.
  6. Scrape cells, pellet at 500 x g for 5 minutes at 4 degrees C.
  7. Snap-freeze the pellet in liquid nitrogen and store at -80 degrees C, or proceed immediately.

Step 2: Cell Lysis and Chromatin Preparation

  1. Resuspend the cell pellet in 1 mL cell lysis buffer. Incubate on ice for 10 minutes.
  2. Centrifuge at 2,500 x g for 5 minutes at 4 degrees C to pellet nuclei.
  3. Resuspend the nuclear pellet in 300 uL nuclear lysis buffer. Incubate on ice for 10 minutes.

Step 3: Chromatin Sonication

This is the most critical and variable step in ChIP-seq. The goal is to shear chromatin to 200-500 bp fragments.

  1. Transfer nuclear lysate to sonication-compatible tubes (e.g., Covaris milliTUBE or Diagenode Bioruptor tubes).
  2. Sonicate using one of these methods:
    • Covaris E220/M220: 140W peak power, 5% duty factor, 200 cycles per burst, 10-20 minutes (optimize for your cell type)
    • Diagenode Bioruptor Plus: 30 sec ON / 30 sec OFF, high power, 15-25 cycles
    • Probe sonicator: 10 sec ON / 30 sec OFF, 30% amplitude, 10-15 cycles on ice (less reproducible)
  3. Centrifuge sonicated chromatin at 14,000 x g for 10 minutes at 4 degrees C to remove debris.
  4. Check fragmentation: Take 20 uL of sonicated chromatin, reverse crosslink (see Step 7), purify DNA, and run on a 1.5% agarose gel or Bioanalyzer. You should see a smear centered at 200-500 bp. If fragments are too large, sonicate more. If too small, reduce sonication time.

Step 4: Immunoprecipitation

  1. Dilute 25-50 ug chromatin (measured by DNA content) in ChIP dilution buffer to reduce the SDS concentration below 0.1%.
  2. Save 5-10% as "input" control (store at -20 degrees C until Step 7).
  3. Pre-clear chromatin with 20 uL Protein A/G beads for 1 hour at 4 degrees C. Remove beads.
  4. Add 2-5 ug ChIP-grade antibody to the pre-cleared chromatin.
  5. Rotate overnight at 4 degrees C.
  6. Add 30 uL Protein A/G magnetic beads. Rotate at 4 degrees C for 2 hours.

Controls:

  • Input: Un-immunoprecipitated chromatin (accounts for chromatin accessibility and sequencing bias)
  • IgG control: Normal rabbit IgG IP (measures non-specific pull-down)

Step 5: Wash the Beads

Perform all washes at 4 degrees C, rotating for 5 minutes per wash:

  1. Low-salt wash buffer — 2 washes
  2. High-salt wash buffer — 1 wash
  3. LiCl wash buffer — 1 wash
  4. TE buffer — 2 washes

Step 6: Elute Immunoprecipitated Chromatin

  1. Add 100 uL elution buffer to the beads.
  2. Incubate at room temperature for 15 minutes with rotation.
  3. Repeat elution and combine eluates (200 uL total).
  4. Also process the input sample (add elution buffer to bring to 200 uL).

Step 7: Reverse Crosslinks and Purify DNA

  1. Add NaCl to a final concentration of 200 mM (8 uL of 5 M NaCl per 200 uL).
  2. Incubate at 65 degrees C for at least 4 hours (or overnight) to reverse formaldehyde crosslinks.
  3. Add 1 uL RNase A (10 mg/mL). Incubate at 37 degrees C for 30 minutes.
  4. Add 2 uL Proteinase K (20 mg/mL). Incubate at 45 degrees C for 2 hours.
  5. Purify DNA using phenol-chloroform extraction followed by ethanol precipitation, or use a PCR purification kit (Qiagen MinElute — elute in 20 uL).

Step 8: Library Preparation

  1. Quantify ChIP DNA using Qubit (fluorometric). Typical yield: 1-10 ng for transcription factors, 10-50 ng for histone marks.
  2. Use the NEBNext Ultra II DNA Library Prep Kit (or equivalent). This kit works with as little as 500 pg input.
  3. Follow the manufacturer's protocol: end repair, adapter ligation, size selection (200-400 bp insert), PCR amplification (8-12 cycles; fewer is better to reduce PCR bias).
  4. Perform size selection with AMPure XP beads (0.8x-1.0x ratio for 200-500 bp fragments).
  5. QC the library on a Bioanalyzer — you should see a peak at 300-500 bp (insert + adapters).

Step 9: Sequencing

  • Read length: Single-end 50 bp is sufficient for most ChIP-seq. Use paired-end 75 bp for better mapping in repetitive regions.
  • Depth: 20-30 million uniquely mapped reads for histone marks; 30-50 million for transcription factors; 10-15 million for input controls.
  • Platform: Illumina NextSeq, NovaSeq, or HiSeq.

Troubleshooting

Low ChIP Enrichment

  • Sonication incomplete: Check fragment size. Over-crosslinked chromatin is harder to sonicate — reduce crosslinking to 8 minutes.
  • Antibody not ChIP-grade: Not all antibodies work for ChIP. Check antibody validation data (ENCODE has validated many ChIP antibodies).
  • Insufficient starting material: Transcription factor ChIP requires more cells (2-5 x 10^7) than histone ChIP.

High Background

  • Insufficient washing: Add an extra high-salt wash or increase rotation time.
  • Too much antibody: Titrate down — more antibody does not always mean better enrichment.
  • Over-sonication: Fragments below 150 bp reduce signal-to-noise.

Poor Library Complexity

  • Too many PCR cycles: Reduce to 8-10 cycles. More cycles increase duplicates.
  • Low input: If ChIP yield is very low, consider using a low-input library prep kit or CUT&RUN/CUT&Tag as alternatives.

Common Mistakes to Avoid

  1. Not checking sonication efficiency — always run a gel before proceeding to IP. Bad fragmentation ruins the entire experiment.
  2. Over-crosslinking — 10 minutes at 1% formaldehyde is standard. More is not better. Over-crosslinking makes sonication difficult and reduces antibody access.
  3. Skipping the input control — input is essential for peak calling. Without it, you cannot distinguish true enrichment from open chromatin bias.
  4. Using too many PCR cycles — library amplification should be minimal (8-12 cycles). Over-amplification creates PCR duplicates that waste sequencing reads.
  5. Insufficient sequencing depth — under-sequenced ChIP-seq produces noisy peaks with low confidence. Follow ENCODE guidelines for depth.

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