B cells were negatively isolated using magnetic beads (EasySep Mouse B Cell Isolation Kit, STEMCELL Technologies). Lysates were clarified from sonicated nuclei and protein-DNA complexes were isolated with antibody conjugated to magnetic beads. Libraries were prepared according to a general protocol for ChIP-seq. Briefly, DNA was end-repaired using End-It DNA End-Repair Kit (Epicentre) followed by a single adenine nucleotide overhang attachment by Klenow fragment (Klenow Fragment 3'>5' exo-, New England Biolabs). The adaptor oligo mix was ligated by T4 DNA Ligase (LigaFast Rapid DNA Ligation System, Promega). Then the adapter ligated DNA was amplified by PCR employing NEBNext High-Fidelity 2X PCR Master Mix (NEB) and a set of indexed primers. Libraries were purified and size-selected using Agencourt AMPure XP beads (Beckman Coulter).