NAT2*27 is assigned uncertain function based on inadequate evidence. NAT2*27 is characterized by the core SNVs c.589C>T and c.803G>A. c.803G>A defines NAT2*4, formerly considered the reference allele, which has repeatedly exhibited increased function in vivo and in vitro. c.589C>T results in the amino acid change p.R197X, introducing a premature stop codon leading to the expression of a truncated protein product that is predicted to be nonfunctional (19164093, 29273096). Four out of four variant effect predictors tested by CPIC staff predict that c.589C>T (p.R197X) impairs function. While in silico predictions indicate that NAT2*27 exhibits decreased or possibly no enzymatic activity, the committee could not come to a consensus given the lack of in vitro or in vivo data to support the function.