An acronym for the beta subunit of a hormone called Human Chorionic Gonadotropin (HCG). This hormone is normally produced during pregnancy. For this reason, the diagnosis of pregnancy can be made by detecting its elevation in the blood. In oncology, it is used as a tumor marker to aid in the diagnosis of certain types of testicular, ovarian or central nervous system tumors. It is usually measured from blood samples, but it can be measured in CSF or urine.