CYP2C19*26 is assigned decreased function based on limited evidence in a heterozygous subject and in vitro studies. CYP2C19*26 was first described in a subject also carrying the CYP2C19*2 allele and demonstrated decreased omeprazole metabolism compared to wildtype and CYP2C19 *1/*2 or CYP2C19*1/*3 subjects based on the plasma metabolite ratio. This result was correlated with an in vitro study which showed CYP2C19*26 demonstrated decreased intrinsic clearance compared to wildtype with mephenytoin and omeprazole substrates (19661214). An additional in vitro study using a different cell system similarly found CYP2C19*26 demonstrated decreased intrinsic clearance compared to wildtype with mephenytoin and clopidogrel substrates (25001882). Therefore, consensus among experts was decreased function based on limited evidence as it was determined the phenotypes containing this allele would be clinically distinct from phenotypes comprising normal function alleles.