Quantification of estrogen receptor–positive (MCF7 and T47D) and triple-negative (Cal51 and MDA-MB-231) breast cancer cell lines as well as nontransformed MCF10A breast cells revealed that, as in primary and metastatic breast cancer, chromosome alignment defects in cells with a clearly established metaphase plate were the most common mitotic defect in the majority of breast cancer cell lines assessed (Fig. 5A–C). This evidence concerns the gene ESR1 and breast cancer.