CYP2C19*3 is assigned no function based on definitive evidence. CYP2C19*3 has been studied for more than 20 years and its association with impaired function is well-established (7969038, 9103550, 22027650). CYP2C19*3 is defined by an early stop codon resulting in a truncated protein lacking a heme binding region and a majority of the substrate binding region, making it a nonfunctional protein. CYP2C19*3 was first identified in a homozygous subject demonstrating decreased mephenytoin metabolism and was subsequently identified in six additional subjects demonstrating decreased mephenytoin metabolism (7969038).