Mature CD4 or CD8 thymocytes were sort-purified for WT or Tcf1/Lef1-deficient mice, the cells were then fixed and sonicated to generate chromatin fragments, followed by immunoprecipitation with antibodies against H3K4me3, H3K27me3, H3K27Ac, H3K9Ac, or Tcf1, which were then properly washed and immunoprecipitated DNA extracted. Splenic CD8 T cells were sort-purified for WT or Tcf1-deficient mice, the cells were then fixed and sonicated to generate chromatin fragments, followed by immunoprecipitation with a Tcf1 antiserum, which were then properly washed and immunoprecipitated DNA extracted. The DNA segments from ChIP were end-repaired and ligated to indexed Illumina adaptors, followed by amplification by PCR with a low number of cells (18-21 cycles). The resulting libraries were seqeunced with Illumina Hiseq-2000 platform. For Histone Marks: ChIPseq, single end, 51 nucleotide sequencing. For several samples we did the high throughput sequencing for a second time to improve the coverage, and the other sequencing strategy is: ChIPseq, single end, 101 nucleotide sequencing. For Tcf1: ChIPseq, single end, 50 nucleotide sequencing.