Progression of the human mammary epithelial cell (HMEC) epigenome during growth. HMECs undergo a brief initial period of exponential growth followed by a temporary growth arrest termed selection/stasis. A subpopulation of vHMECs is able to escape this senescence and exhibit a second, longer period of exponential growth before becoming permanently arrested at agonescence. Much like during cancer progression, HMECs undergo a stepwise change in DNA methylation levels at selection/stasis and then again following the forced oncogene-induced escape from agonescence. vHMEC, variant human mammary epithelial cell.