Progressive atypical histology of the PSI in Cyp1a1(−/−) knockout mice (following 12 weeks of oral BaP, 12.5 mg/kg/day), showing normal intestinal villi together with nests of neoplastic cells near the basement membrane. A, area of normal proximal small intestine. (B, C, & D) areas involved by a progression from epithelial dysplasia to carcinoma in situ (white arrows). (E & F) unequivocal areas of invasive carcinoma into the stroma and the muscle layer (white arrows). Inset: a small region of metastasis (white arrow) found in a nearby lymph node. Few (if any) lymphocytes or other immune cells implicating possible inflammation are present. Bar, upper right = 100 microns for all images.