CYP2C9 and toxic epidermal necrolysis: For phenytoin, 23andMe reports on drug sensitivity from CYP2C9*2 and *3 variants, but the U.S. FDA additionally lists pharmacogenetic information on HLA-B*15:02, which confers increased risk of life threatening hypersensivity reactions like Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) [12].