No discernable effect of rat strain on apoptosis in the mammary gland. A, Representative western blots of protein lysates prepared from mammary glands of ACI and BN rats, treated with E2 for 1, 3 or 12 weeks, and probed with antibody to cleaved forms (19 kDa and 17 kDa fragments) of caspase 3 (n = 3). B, The amount of cleaved caspase 3 was quantified using a Bio-Rad ChemiDoc XRS + imaging system and normalized to the amount of β-actin. Each data bar represents the mean ± SEM, n = 3 biological replicates. C, Cells isolated from mammary glands of ACI and BN rats, treated with E2 for 3 weeks, were stained with Annexin V and propidium iodide and analyzed by flow cytometry. Cells isolated from an involuting mammary gland (3 days post-lactation) were stained and analyzed as a positive control. D, Each data bar represents the number of apoptotic cells (positive for annexin V, negative for PI) expressed as a percentage of total PI negative cells (mean ± SEM, n = 3).