CD8 T cells were then crosslinked, sonicated with Covaris instrument, and ChIP was done with either Tcf1 antibody or control IgG. For control IgG, pre-immune rabbit serum that we obtained from a different antibody production project was used to match the anti-Tcf1 (which is unfractionated rabbit antiserum rather than IgG).The immunoprecipitated fragments were reverse-crosslinked and purified. chromatin DNA fragments were end-repaired, and ligated with Illumina adaptors. The DNA was then PCR amplified with for 18 cycles and library fragments of 220-400 bp were purified from gel. The purified DNA was captured on an Illumina flow cell for cluster generation. Libraries were sequenced on the Genome Analyzer following the manufacturer's protocols.