To purify epidermal cells, back skins from mice were incubated for 1 h in 0.25 % trypsin at 37 C. Trypsin activity was neutralized by the addition of 10% chelated FBS Ca2+-free EMEM, followed by mechanical dissociation and sequential filtration through 100 µm and 40 µm cell strainers (SPL Life Sciences). Library preparation for ATAC sequencing used 50,000 IFE or bulge stem cells FACS-sorted from three independent mice as previously described in Buenrostro et al., Nat. Methods 2013