Genomic DNA isolated from naïve and activated B cells was spiked with unmethylated lambda phage cI857 Sam7 DNA (Promega, Madison, WI) and a PCR amplicon from puromycin resistant gene at a ratio of 200:1 and 100,000:1, respectively. DNA (5-10 ug in 130 uL TE buffer) was sheared with a Covaris E220 (Covaris) using microTUBE for 4 mins. DNA was cleaned-up with AmureXP beads, processed with NEBNext End Repair and A-tail Modules (NEB, Ipswich, MA), and ligated to methylated Illumina adaptors (NEB). DNA was then bisulfite-treated, denatured, and immunoprecipitated with anti-CMS serum (in house) and mixture of protein A and G dynabeads (ThermoFisher). Libraries for immunoprecipitated DNA were generated by PCR with barcoded primers (NEBNext Multiplex Oligos for Illumina, NEB) for 15 cycles using KAPA HiFi HotStart Uracil+ ReadyMix (Roche), followed by a cleanup with AmpureXP beads (Beckman Coulter). Bisulfite-seq, 50bp paired-end with HiSeq 2500