CDKN2A and breast cancer: We first assessed the ability of MCF10A (non-transformed, immortalized mammary epithelial cells lacking the cyclin kinase inhibitors p16INK4A and p15INK4B due to a homozygous deletion), ER-PR-HER2- and ER-PR-HER2+ breast cancer cells to form colonies after increasing single doses of γ-radiation (0, 1, 2, 4, 6, or 8 Gy).