ATAC-Seq was performed as previously described22 (link) on 5,000–40,000 FACS-purified cells from 2–9 mice. In brief, cells were lysed in lysis buffer for 1 minute and transposed with Tagment DNA Enzyme 1 (Illumina) for 30 minutes. DNA was cleaned up using a MinElute DNA purification Kit (Qiagen), followed by barcoding and library preparation by the Nextera DNA Library preparation kit (Illumina) according to manufacturer’s guidelines and sequenced on an Illumina NextSeq500.