CYP2C19*4 is assigned no function based on limited evidence in heterozygous subjects and an in vitro study. Two subjects with CYP2C19 *2/*4 genotype demonstrated decreased mephenytoin metabolism compared to wildtype subjects based on urine metabolite concentrations. This correlated with the in vitro experiment which found CYP2C19*4 protein was not expressed but mRNA levels were similar to wildtype indicating CYP2C19*4, which is defined by a SNP in the initiation codon, is not properly translated (9435198). Thus, the CYP2C19*4 defining SNP would result in a nonfunctional protein, even in the presence of a SNP associated with increased enzyme function (21358751). Therefore, consensus among experts was no function based on limited evidence as it was determined the phenotypes containing this allele would be clinically distinct from phenotypes comprising normal function and decreased function alleles.