NAT2*4 is assigned increased function based on definitive evidence. The NAT gene nomenclature committee considered NAT2*4 as the reference allele for over two decades before the transfer of NAT2 allele nomenclature to PharmVar, which uses the GRCh38 (NC_000008.11) reference sequence. The NAT gene nomenclature committee has historically assigned acetylation activity to alleles as either "rapid" or "slow", classifying the reference NAT2*4 allele as "rapid". Under the CPIC allele function terminology, rapid acetylation alleles are categorized as "Increased Function". NAT2*4 has been studied for more than 25 years and numerous clinical and in vitro studies have demonstrated rapid acetylation by NAT2*4 with various substrates. Early clinical studies demonstrate rapid acetylation of caffeine metabolites in individuals homo- or heterozygous for the NAT2*4 allele (8102597, 7668286, 8923605, 9891107). Additional clinical studies demonstrate rapid acetylation by NAT2*4 using isoniazid (2376572, 9321764, 15565348) and sulfamethazine (1872889, 21946899). Individuals homo- or heterozygous for NAT2*4 metabolize hydralazine rapidly (24702251, 31257615). NAT2*4 is distinguished from NAT2*1 by the presence of the core SNV c.803G>A (p.R268K). c.803G>A (p.R268K) has not been shown experimentally to alter NAT2 acetylation activity in vitro nor in vivo. Accordingly, eight out of eight variant effect predictors tested by CPIC staff predict that c.803G>A is neutral/tolerated for protein function.