ATAC-seq was performed on 10,000 sorted progenitor B-cells as previously described (Buenrostro J D, et. al 2015. Current Protocols in Molecular Biology 109: 1-9). Sorted cells were permeabilized and exposed to 0.5µl transposase. ATAC-seq libraries were constructed using the Nextera DNA library Prep kit (Illumina) according to the instructions by the manufacturer, and sequencing was performed on a HiSeq 1500 Illumina machine.