Arylamine N-acetyltransferase 1 (NAT1) is a ubiquitously expressed phase II drug metabolizing enzyme responsible for the metabolism of arylamine and hydrazine compounds. Historically, its role in cancer has been linked to its bioactivation and detoxification of carcinogenics. However, recent studies have shown that NAT1 may have a more intimate role in cancer biology. In particular, studies in HT-29 colon cancer cells and MDA-MB-231 breast cancer cells suggest that NAT1 supports cancer cell survival and growth [1, 2]. NAT1 knockdown in these cells decreases cell proliferation, possibly by altering epigenetic regulation. This was investigated further by metabolomic analyses of the s-adenosylmethionine (SAM) cycle in HT-29 control and NAT1 knockdown cells, since the methyl group for DNA methylation comes from SAM. While the results provided no evidence linking NAT1 knockdown with altered SAM cycle function, investigation of the cell culture medium indicated that NAT1 supported the growth of control cells in a methionine independent manner. Further studies have shown that NAT1 appears to be necessary for a functional methionine salvage pathway in HT-29 cells, a pathway that has been implicated in several different cancers [3-5]. We propose that NAT1 moonlights as an enzyme in the methionine salvage pathway and performs the role of the methylthioribose isomerase MRI1.