Already in the 1990s, Pogo et al. [134] tried to investigate the association of MMTV-positive BC with c-erbB-2, p53, bcl-2, progesterone receptors, and cathepsin D and had tried to classify MMTV-positive tumors within one of the two subsets of BC known at that time [135]. The gene discussed is CTSD; the disease is breast cancer.