Finally, a new generation of AIs, including aromatase non-steroidal, reversible inhibitors, as Anastrozole and Letrozole and a steroidal, irreversible aromatase inhibitor, Exemestane, were developed in the mid-1990s, and radically changed the outcome of post-menopausal hormone-sensitive women with BC [16]. This evidence concerns the gene CYP19A1 and breast cancer.