ESR1 and breast cancer: In the assessment of the molecular BC subtype, a higher incidence of the mutation was found in the patients with a progesterone receptor-negative (PR−) BC (0.55% vs. 0.24%, p = 0.03), estrogen receptor-negative (ER−) BC (0.43% vs. 0.29%) and TNBC (0.54% vs. 0.33%) than in the corresponding patients with a receptor-positive BC.