Compositions and methods are provided for the diagnosis, treatment, and medical management of cancers. The methods include the use of antibodies and other binding molecules that specifically bind to one or more nucleotide salvage pathway enzymes (SPEs) selected from the group consisting of adenine phosphoribosyltransferase (APRT), hypoxanthine-guanine phosphoribosyltransferase (HGPRT), deoxycytidine kinase (dCK); and thymidine kinase 1 (TK1) and complexes comprising SPEs, for detection of the SPE(s) on or in cancer cells and/or on or in body fluids and tissues of cancer patients. Binding of SPEs is useful in the methods provided herein for diagnosing cancer, determining prognosis of cancer and assessing the effectiveness of cancer treatments. Immunoassay systems for use in the methods are also provided, including sandwich immunoassays. In addition, an amino acid sequence comprising a novel TK1 binding site is provided, as well as nucleotide sequences encoding it.