CYP2C9*13 is defined by a missense variant in exon 2 (NM_000771.3:c.269T>C, p.Leu90Pro, rs72558187). CYP2C9*13 prevalence is approximately 1% in the Asian population, but in Caucasians this variant prevalence is almost zero. This variant is caused by a T269C mutation in the CYP2C9 gene which in turn results in the substitution of leucine at position-90 with proline (L90P) at the product enzyme protein. This residue is near the access point for substrates and the L90P mutation causes lower affinity and hence slower metabolism of several drugs that are metabolized CYP2C9 by such as diclofenac and flurbiprofen. However, this variant is not included in the tier 1 recommendations of the PGx Working Group because of its very low multiethnic minor allele frequency and a lack of currently available reference materials. As of 2020, the evidence level for CYP2C9*13 in the PharmVar database is limited, comparing to the tier 1 alleles, for which the evidence level is definitive.