Epigentek Home
EpiGentek Guarantee

Cross-linking ChIP (X ChIP) Protocol


Cross-linking ChIP Protocol

ChIP is an immunoprecipitation technique used to study interactions between proteins and DNA in a cell. The goal is to determine what specific proteins are associated with different regions in the genome. The target protein is cross-linked together with DNA, and then the DNA is broken into smaller fragments with sonication or enzymatic hydrolysis. The DNA fractions of interest can be pulled down by specific antibody-antigen reactions. Therefore, this process enriches for the DNA fragments bound by target proteins. Finally, DNA can be purified from the complex and analyzed with PCR or qPCR protocols.

Cross-linking ChIP Diagram

X ChIP diagram courtesy of He C. and Bonasio R. Click to enlarge.


Solution and reagents

ChIP lysis buffer:

1% TritonX-100, 0.1% NaDOC, 0.1% SDS, 1mM EDTA (pH8.0), 140mM NaCl, 50mM Tris-HCl (pH 8.0)

RIPA buffer:

1% NP-40, 0.5%NaDOC, 0.1% SDS, 2mM EDTA (pH8.0), 150mM NaCl, 50mM Tris-HCl (pH 8.0)

Low salt wash buffer:

1% TritonX-100, 0.1% SDS, 2mM EDTA (pH8.0), 150mM NaCl, 20mM Tris-HCl (pH 8.0)

High salt wash buffer:

1% TritonX-100, 0.1% SDS, 2mM EDTA (pH8.0), 500mM NaCl, 20mM Tris-HCl (pH 8.0)

LiCl buffer:

0.25M LiCl, 1% NP-40, 1% NaDOC, 1mM EDTA, 10mM Tris-HCl (pH 8.0)

TE buffer:

1mM EDTA, 10mM Tris-HCl (pH 8.0)

Elution buffer:

1% SDS, 100mM NaHCO3


Sample preparation

Cross-linking and lysis - for transcription factor and cofactors

Use cell culture from 10cm culture dishes with 20mL of culture medium. Cells are ready to be used for a ChIP assay when the density of cells reaches 80%-90%. The abundance of target protein depends upon the protein of interest and the number of cells. Refer to the table below:


 Number of cells (for a ChIP reaction)

 Protein target abundance

Histone protein
RNA polymerase II



Transcription factor




107 or more



For optimal results, a starting concentration of >4×106 cells for histone protein and RNA polymerase II is recommended. Transcription factors and cofactors may require more cells. 1% formaldehyde is used to crosslink proteins and DNA. This process is time-dependent and needs to be optimized for different types of cells. Crosslinking deficiency may result in a false negative, while excessive crosslinking may cause reactivity with non-specific epitopes and yield a false-positive result. Therefore, crosslinking the sample for 10min is recommended. The crosslinking reaction can be reversed by glycine.

  1. To prepare a 1% formaldehyde solution, take out the culture dishes and add 550μl of 37% formaldehyde into the 20ml of culture medium. Mix the medium.
  2. Place the culture dishes at RT for 10 min.
  3. Add 125mM glycine solution to terminate the cross-link reaction. Gently mix the solution.
  4. Incubate the culture dishes at RT for 5 min.
  5. Remove culture medium and wash the cells with ice-cold PBS three times.
  6. Add 1ml of ice-cold PBS, into the dishes and scrape the cells from the dishes quickly.
  7. Wash the bottom of dishes with PBS for 2 times with proper volume. Aspirate the PBS into the tube from step 6.
  8. Centrifuge for 3min, 4℃ at 2500 RPM to collect the pellets.
  9. Add the appropriate volume of ChIP lysis buffer (1ml for every 2×107 cells) to suspend the cells. Next, lysate the cells on ice for 15 min


Sonication conditions may differ depending on the sonicator and type of cells used. Depending on the different cell lines, the ideal fragment size of DNA after sonication is 200-1000bp. See our Sonication Application protocol to determine the ideal size for your experiment.

  1. Sonicate the cells to obtain the ideal fractions of DNA is 200-1000bp. Sonication conditions may differ depending on the sonicator or type of cells used.
  2. Centrifuge for 10min, 4℃ at 12000 RPM. Transfer the supernatant into a new tube.
  3.  Use 50μl of supernatant for an agarose gel analysis to validate the effect of sonication.

Determination of DNA fragment size

  1. Add 70μl elution buffer to the 50μl of chromatin.
  2. Add 4.8μl 5M NaCl and 2μl of 10mg/ml RNase A, then incubate at 65℃ overnight. The purpose is to remove interference from RNA in the sample.
  3. The next day, add 2μl of 20mg/ml proteinase K and incubate at 60℃ for 1 hour. The aim is to break the reactions between protein and DNA so that the DNA can be purified.
  4. Use a DNA purification kit to enrich the DNA (or perform a phenol-chloroform extraction and ethanol precipitation method).
  5. A 2% agarose gel can be prepared to detect the efficiency of sonication.


  1. Transfer 500μl of chromatin (diluted by RIPA buffer) into a new tube and use 50μl chromatin as an input amount .
  2. Prepare four groups of sample conditions :
    Experimental group: add 2-5μg of specific antibody into 500μl cell lysate and place tube on a rotary mixer at 4℃ overnight to form the antigen-antibody complex.
    Input group: follow the same procedure as the experimental group. However, do not add antibodies.
    Negative control: follow the same procedure as the experimental group, but add a rabbit IgG antibody control.
    Positive control: follow the same procedure as the experimental group but add a histone H3 or RNA polymerase antibody control.
  3. The next day, add 50μl of magnetic beads to the complex solution and incubate at 4 oC for 2 to 4 hours.
  4. Place tube on magnetic stand and remove the supernatant.

Wash the beads

Wash the beads with filtered tips:

  1. 2×1ml Low salt wash buffer
  2. 2×1ml High salt wash buffer
  3. 2×1ml LiCl buffer
  4. 2× 1ml TE buffer

Wash the beads two times with the buffers above. Pipette 3-8 times by tips and rotate for 10 min each. Remove wash buffer by placing tubes on magnetic stand.

Reverse crosslinking

  1. Add 120μl Elution buffer to the precipitation complex and gently mix. Place tubes on rotating mixer at RT for 15 min. Repeat this step once more for a total of two times.
  2. Place tubes on magnetic stand and transfer the supernatant to a new tube.
  3. Add 9.6μl of 5M NaCl and 2μl of 10mg/ml RNase into all sample groups. Incubate at 65℃ overnight to reverse crosslinkage.
  4. The next day, add 2μl of 20mg/ml protein K and incubate at 60℃ for 1 hour.

DNA purification

DNA can be extracted by using the EpiNext DNA Purification HT System and following the procedure found in the user guide.


Perform a PCR or Real-time PCR assay to analyze the DNA associated with the target protein.

Terms & Conditions | Privacy Policy | Site Map
Copyright © 2019 EpiGentek Group Inc. All rights reserved.