CYP2C19*5 is assigned no function based on moderate evidence in heterozygous subjects and in vitro studies. CYP2C19*5 was first described in a subject demonstrating decreased mephenytoin metabolism compared to wildtype subjects and similar mephenytoin metabolism compared to CYP2C19 *2/*2, CYP2C19 *2/*3, and CYP2C19 *3/*3 subjects based on S/R mephenytoin ratio in urine metabolites (9103550) . One CYP2C19*2/*5 subject similarly demonstrated decreased phenytoin metabolism based on S/R mephenytoin ratio in urine metabolites. This result was correlated with an in vitro study which showed CYP2C19*5 had a 40% decrease in enzyme expression, 0.4% enzyme activity with a mephenytoin substrate, and nondetectable enzyme activity with a tolbutamide substrate compared to wildtype (10022751). Two additional in vitro studies using different cell systems found CYP2C19*5 had nondetectable enzyme activity with mephenytoin, omeprazole, and clopidogrel substrates (21325430, 25001882). CYP2C19.5B showed no activity towards CEC (3-cyano-7-ethoxycoumarin) O-deethylation (21692664). Therefore, consensus among experts was no function based on moderate evidence.