Numbers in the names of tumor markers
There are dozens of cell marker genes. The number refers to antigen protein from one of those specific genes. The number itself can be a result of any of a variety of things, from the order in which the protein was discovered to the corresponding antibody being the 125th antibody produced against the cell line or organism. These are often used as biomarkers of various cancers. CA-125, for example, is sometimes found at high levels on the surface of blood cells in women with ovarian cancer.