In Vitro: 1-Ethynylnaphthalene, selective inhibitor of cytochrome P450 IB1, does not affect 2-hydroxylation but inhibits 4-hydroxylation by 38%. At higher concentrations, 1-Ethynylnaphthalene inhibits 2-hydroxylation of 2-Hydroxylation of estradiol (E2) by ~30% and 4-hydroxylation of E2 by up to 80%.