Bromodeoxyuridine-labeling index (number labeled per 500 cells) in hormone-treated p53-null mammary epithelial transplants. Transplants from each experiment (1, 2 and 3) were assayed for bromodeoxyuridine. The black bars represent untreated transplants, the gray bars hormone-treated transplants. The number above each pair of bars indicates the number of weeks after the removal of hormones. In experiment 3, 12 weeks after the removal of hormone represents 8 weeks after transplantation. There were four transplants per treatment group. Five hundred cells were counted in each transplant. All four comparisons were significantly different (p < 0.05).