NAT2*56 is assigned decreased function based on moderate evidence. NAT2*56 is characterized by the core SNVs c.58G>T (p.D20Y), c.590G>A (p.R197Q) and c.803G>A (p.R268K). c.803G>A (p.R268K) defines NAT2*4, formerly considered the reference allele, which has repeatedly shown increased function in vivo and in vitro. There is no literature describing function for c.58G>T (p.D20Y). Eight out of nine variant effect predictors tested by CPIC staff predict that c.58G>T (p.D20Y) impairs protein function. c.590G>A (p.R197Q) defines NAT2*6 which has been extensively studied, exhibiting decreased N-acetyltransferase activity both in vivo and in vitro. Similarly, seven out of nine variant effect predictors tested by CPIC staff predict that c.590G>A (p.R197Q) impairs protein function. Thus, NAT2*56 is assigned decreased function based on c.590G>A. Despite this, the combinatorial effects associated with c.58G>T and c.590G>A are uncertain.