HAT Medium (hypoxanthine-aminopterin-thymidine medium) is used in mammalian cell culture for post-fusion selection to eliminate certain types of myeloma cells

The combination of aminopterin, a drug that acts as a powerful folate metabolism inhibitor, with hyproxanthine and thymidine, which are intermediates in DNA synthesis, provides a form of artificial selection for cells containing functioning hypoxanthine-guanine phosphoribosyltransferase (HGPRT) and thymidine kinase (TK). These enzymes are involved in DNA synthesis and cell division. HAT medium is often used in the preparation of monoclonal antibodies.