It can be classified as luminal A, which has a low proliferative rate (measured by Ki67 protein expression) and is positive for estrogen receptor (ER) and/or progesterone receptor (PR); luminal B, which has a high Ki67 expression and is ER and/or PR positive; HER-2 positive, which has a high level of human epidermal growth factor receptor HER-2 amplification/expression; or a heterogenous group of tumours characterised by the absence of ER, PR, and HER-2 that are classified as triple-negative breast cancer (TNBC) [3,4,5]. This evidence concerns the gene ESR1 and neoplasm.