They also observed that the MMR-deficient breast cancers had pathology features that were consistent with the features identified in colorectal cancers caused by MMR gene mutations [66,67], that is, MMR-deficient breast cancers were more likely to be poorly differentiated, have a higher mitotic rate, peritumoral lymphocytes, confluent necrosis, a solid growth pattern, and more likely to be estrogen receptor-negative and progesterone receptor-negative than the MMR-proficient breast cancers. The gene discussed is ESR1; the disease is breast cancer.