Only in case of tumors, the disruption of the normal tissue architecture due to the growing tumor allows MUC1 mucin to be shed into the circulation where it can be measured via immunoassays kits to detect cancer antigen 15.3 (CA 15.3), mucin-like carcinoma associated antigen, breast cancer mucin, and the like. The gene discussed is MUC1; the disease is neoplasm.