The effect of duration of hormone exposure on tumorigenesis in p53-null mammary epithelial transplants. The effect of hormone exposure was compared with that of a hormone receptor antagonist, tamoxifen. Mice treated with estradiol and progesterone, either early or late, or treated with tamoxifen, had a significant decrease in tumor incidence (p < 0.05). Filled circles and solid line, control; filled circles and dotted line, early hormone exposure; inverted triangles and broken line, late hormone exposure; upright triangle, tamoxifen. The flow diagram illustrates the experimental treatment plan.