CYP2C19*12 is assigned uncertain function due to inadequate evidence. In one in vitro study CYP2C19*12 was poorly expressed and enzyme activity could not be assessed with a mephenytoin substrate (12464799). CYP2C19*12 is defined by the conversion of the stop codon to cysteine, and it is unclear if this protein is unstable in vivo. Therefore, consensus among experts was not achieved and uncertain function was assigned.