NAT2*19 is assigned decreased function based on limited evidence. The NAT gene nomenclature committee has historically assigned acetylation activity to alleles as either "rapid" or "slow", classifying the NAT2*19 allele as "slow" . Under the CPIC allele function terminology, slow acetylation alleles are categorized as "Decreased Function". NAT2*19 is characterized by the core SNVs c.190C>T (p.R64W) and c.803G>A (p.R268K). c.803G>A defines NAT2*4, formerly considered the reference allele, which has repeatedly exhibited increased function in vivo and in vitro. In vitro studies of c.190C>T (p.R64W) have shown decreased protein expression, decreased thermostability, and significantly decreased N-acetyltransferase activity toward sulfamethazine (12222688). An additional in vitro study found that amino acid substitutions of arginine 64 to alanine, lysine, methionine, or glutamine (the latter of which defines NAT2*14) significantly decreased NAT2 protein stability and N-acetyltransferase activity toward p-aminosalicylic acid and all but p.R64K showed decreased N-acetyltransferase activity toward sulfamethazine (9173883). These results highlight the importance of the p.R64 residue for proper NAT2 catalytic activity. Published in silico modeling predicts that c.190C>T (p.R64W) decreases NAT2 protein stability, decreases flexibility and hydrogen bond capacity, and increases hydrophobicity and aromaticity resulting in a loss of electrostatic interactions (18680467). Similarly, six out of nine variant effect predictors tested by CPIC staff predict that c.190C>T (p.R64W) impairs NAT2 protein function.