ESR1 and neoplasm: The diagnosis and prognostication of BCs are rooted in the interlacing of traditional clinico-pathological variables (i.e., tumor size, grade, lymph node positivity, age) with immunohistological benchmarks for proliferation index (Ki67), hormone receptor status [i.e., expression of estrogen receptor (ER) and progesterone receptor (PR)], and human epidermal growth factor receptor (HER2) levels.