For bone marrow subset purification we used B6.RAG1-/-mice to obtain pro-B cells. B6.RAG-/-+ mice bearing a rearranged IgM transgene were used to obtain pre-B cells. For pro-B IL7, RAG1-/- pro-B cells were cultured with IL7 for 7 days. For each sample 1 - 10 ng of IP material or input control sample was prepared into sequencing libraries using the NEBNext Ultra DNA Library Prep Kit for Illumina following manufacturers protocol. Samples were amplified using 18 cycles of PCR and then size selected on 2% EX agarose gels (Thermo Fisher) and bands cut at 300bps (+ 25 bps) and cleaned up using DNA Clean & Concentrator-25 (Zymo Research). The prepared libraries were loaded onto an Illumina HiSeq2000 for single-end base reads with 7 bases of the index read. Data was processed to generate Fastq files using CASAVA 1.8 and demultiplexed based on index sequences.