HAT Supplement is a liquid mixture of sodium hypoxanthine, aminopterin and thymidine.  After dilution, HAT is suitable for use as a post-fusion selective medium to eliminate unfused or self-fused HGPRT-myeloma cells.  Hypoxanthine and thymidine supply preformed purines and pyrimidines for DNA synthesis by hybridomas via the salvage pathway that utilizes HGPRT- contributed by the fused spleen cell.  Aminopterin, a folic acid antagonist, inhibits the de novo nucleoside biosynthesis pathway.