NAT2*34 is assigned decreased function based on definitive evidence. The NAT gene nomenclature committee has historically assigned acetylation activity to alleles as either "rapid" or "slow", classifying the NAT2*34 allele as "slow" . Under the CPIC allele function terminology, slow acetylation alleles are categorized as "Decreased Function". NAT2*34 is characterized by the core SNV c.590G>A (p.R197Q) which has been studied for more than 25 years and has shown decreased NAT2 N-acetylation activity in numerous in vivo and in vitro studies using various substrates. In vivo phenotyping studies have demonstrated that individuals heterozygous for NAT2*34 in combination with another decreased function allele exhibit decreased N-acetylation activity toward caffeine metabolites (15558239, 17011540) and isoniazid (9154882, 28959610). Seven out of nine variant effect predictors tested by CPIC staff predict that c.590G>A (p.R197Q) impairs protein function. In vitro studies have shown that NAT2*34 exhibits decreased thermostability and decreased N-acetyltransferase activity toward sulfamethazine and isoniazid (28523442).