Radiation of different doses and qualities promotes outgrowth of variant human mammary epithelial cells. (A) Human mammary epithelial cells (HMEC) cultures (10 or more replicates per dose) from specimen B1450 were subjected to X-ray doses between 5 and 200 cGy. Each treatment group was normalized to a control group and the resulting means ± SE were plotted versus dose. (B and C) When assayed four to six weeks after irradiation with indicated doses (cGy) of 56Fe or γ-rays, indicated HMEC cultures showed statistically significant differences (P < 0.0001, non-parametric Wilcoxon test) in the percentages of (B) SA-βgal (-) and (C) BrdU (+) cells compared to control cultures. Scatter plots indicate the mean ± SE of between 5 and 10 independent replicates for three individual specimens assayed for each treatment.