ABSTRACT: Reports indicate that toxic equivalency factors (TEFs) based primarily on rodent data do not accurately predict in vitro human responsiveness to certain dioxin-like chemicals (DLCs). To investigate this in cells responsive to dioxins and relevant to chloracne, normal human epidermal keratinocytes were treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and several DLCs, each with a TEF value of 0.1, representing three classes of congeners. We estimated half maximal effective concentration (EC50)-based donor-specific relative potency (REP) values for cytochrome P450 1A1 (CYP1A1) messenger RNA (mRNA) induction for TCDD, 1,2,3,6,7,8-hexachlorodibenzo-p-dioxin (HxCDD), 2,3,7,8-tetrachlorodibenzofuran (TCDF), 1,2,3,6,7,8-hexachlorodibenzofuran (HxCDF), and 3,3',4,4',5-pentachlorobiphenyl (P
