Mammary glands were dissociated and cells purified using previously established protocols (PMID: 20445538). Lysates were extracted from sorted nuclei using a nuclei lysis buffer (according to ATAC-seq protocol from Buenrostro et al 2013). Transposase reaction was carried out following the protocol of the Nextera DNA library preparation kit (illumina). Libraries were prepared using the NeBNext buffer and appropriate ATAC-seq libraries adaptors (Illumina, Buenrostro et al, 2013). The llibrary was size-selected using the DNA on a ChIP technology.