CYP2C19*9 is assigned decreased function based on limited evidence. Three in vitro studies using different cell systems found CYP2C19*9 demonstrated decreased enzyme activity compared to wildtype with mephenytoin, omeprazole, and clopidogrel substrates (12464799, 21325430, 25001882). Another study found CYP2C19*9 exhibited significantly reduced catalytic activities compared with CYP2C19*1 across multiple substrates (21692664).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.