Skeletal muscle biopsies from patients diagnosed with BC (n = 33) and non-cancer controls (n = 10) were used for RNA-seq, and BC patients were classified based on molecular subtype of their primary tumor, as follows: luminal (ERPR)—positive for ER and PR without overexpression of HER2/neu; HER2—overexpression of HER2/neu in the absence of ER and PR expression; triple negative (TN)—absence of ER, PR, and HER2/neu expression; and triple positive (TP)—presence of ER and PR expression, and overexpression of HER2/neu (ERPR n = 10, HER2 n = 5, TN n = 9, TP n = 9). This evidence concerns the gene ESR1 and breast cancer.