To create "humanized" CYP1A1 and CYP1A2 transgenic mice (hCYP1A1_1A2), a single copy of the 180-kb human CYP1A1_CYP1A2 locus-containing BAC-H (BAC Human CTB clone 31H21: including the 23.3 kb spacer region, 90 kb of CYP1A2 3'-flanking region and 53 kb of CYP1A1 3' flanking region) was microinjected into (C57BL/6J x DBA/2J)F1 oocytes. Founder mice having a single copy of BAC-H were identified and then backcrossed for 10 generations to C57BL/6 mice. (J:96716)