Splenic B cells isolated using the Stem Cell Technologies EasySep Mouse B cell isolation kit were rested for 1-2 hours in complete media (RPMI-1640 plus 10% fetal bovine serum, 1X Glutamax, 1X penicillin-streptomycin and 50 micromolar beta-mercaptoethanol) in a tissue culture incubator at 37C. After resting, cells were crosslinked with 0.25% formaldehyde for 8 minutes. Fixed B cells were lysed, chromatin were prepared and immunoprecipitated according to the protocol of ChIP-IT High Sensitivity Kit (Active Motif). ChIP-seq libraries were prepared using the manufracturer's protocol of Rubicon ThruPlex kit