We used insertional ChIP (iChIP) based approach to identify proteins that bind to mouse immunoglobulin switch (S) region. To perform iChIP, we inserted an 8X-repeat of the LexA-binding element (LexA-BE) downstream of the Sα region in CH12F3-2A cells, a mouse B-cell line. The DNA-binding domain and dimerization domain of the LexA protein fused with a FLAG tag and a nuclear localization signal (NLS) was expressed in WT (FNLDD-alone) or Sα-engineered (FNLDD-Sα-LexA) CH12F3-2A cells. The resultant cells were subjected to stable isotope labeling with amino acids in cell culture (SILAC), stimulated with CIT (CD40L, IL4, and TGFβ), immunoprecipitated with an anti-FLAG antibody, and subjected to LC-MS/MS analysis. The identities of the deposited data are as follows: F: WT (FNLDD-alone); D: Sα-engineered (FNLDD-Sα-LexA).