Activation of EGFR tyrosine kinase inhibitor resistance characterizes most breast cancer patients also [43], along with the overexpression of CYP19A1, an enzyme with a key role in the steroid hormone biosynthesis, metabolizing testosterone into estradiol and androstenedione to estrone, whose dysregulation is relevant to estrogen-dependent pathologies [44]. This evidence concerns the gene CYP19A1 and breast cancer.