CYP2C19*6 is assigned no function based on limited evidence in a heterozygous subject and in vitro studies. CYP2C19*6 was first described in a subject demonstrating decreased mephenytoin metabolism based on urine metabolite concentrations. This correlated with an in vitro study which found CYP2C19*6 had nondetectable enzyme activity with mephenytoin and tolbutamide substrates (9732415). Two additional in vitro studies using different cell systems found CYP2C19*6 had nondetectable enzyme activity with mephenytoin, omeprazole, and clopidogrel substrates (21325430, 25001882). 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.