ChIP was performed as described previously(87 (link)). Briefly, ~107 cells were cross-linked with 1% methanol-free formaldehyde for 10 min at room temperature. The fixation was quenched by adding glycine to a final concentration of 125 mM and placed on ice for 5 minutes. Fixed cells were pelleted at 1,200 × g for 5 minutes at 4°C and resuspended in ice-cold PBS containing a protease inhibitor cocktail. Samples were sonicated with an Active Motif EpiShear probe sonicator. After sonication, a 5μL volume was aliquoted from each sample (input) and combined with 20μL TE, 1μL 10% SDS, and 1μL 20 mg/mL Proteinase K for overnight decrosslinking at 65°C. Input samples were purified using Qiagen MinElute PCR Purification columns, and chromatin fragmentation was assessed by gel electrophoresis on an E-Gel 2% EX agarose gel. The cleared chromatin solutions were immunoprecipitated with antibodies against H3K4me3 (ab8580; Abcam). The normal rabbit IgG (ab171870; Abcam) was used as a background control. Immunoprecipitated complexes were isolated with protein A Dynabeads. ChIP DNA was purified with Qiagen MinElute PCR Purification columns. The DNA was used for ChIP-qPCR with specific Prdm1 PCR primers (supplemental Table 5). BIO-RAD CFX96™ Real-Time PCR Detection System was used for SYBR green-based quantitative PCR. The data are presented as fold enrichment over the input control.