PHT is primarily metabolized by liver cytochrome P450 enzymes, particularly CYP2C9 and CYP2C19 [4] to form enantiomers of 5-(4-hydroxyphenyl-),5-phenylhydantoin (HPPH) which in addition to PHT, have been implicated in the pathogenesis of DIGO [5,6]. The gene discussed is CYP2C9; the disease is pulmonary hypertension, primary, 1.