CD13/aminopeptide N (APN, EC 3.4.11.2) is a 150 kDa zinc-dependent type II metalloprotease that is expressed on various tissues and cell types, particularly on the membranes of tumor cells, such as multiple myeloma, liver cancer, melanoma, ovarian cancer, prostate cancer, colon cancer, pancreatic cancer, breast cancer, and lung cancer cells [1,2,3]. The gene discussed is ANPEP; the disease is breast cancer.