Female mouse C57BL/6 B cell populations were sorted on BD FACSAriaII, BD Influx or Beckman Coulter MoFlo XDP cell sorters; collected cells were centrifuged and cross-linked with 1% formaldehyde and sonicated to shear the chromatin to ~500bp fragments; DNA was isolated with antibodies and purified by phenol-chloroform extraction Libraries were prepared with Illumina's DNA Sample prep kit according to the manufacturer's instructions