Fig. 1. The majority of patched mutant mice have ectopic cells in their cerebellum before they develop tumors. Cerebella from a 6-week-old wild-type mouse (A), a 6-week-old patched^+/– mouse (B) and a 12-week-old patched^+/– mouse with a tumor (C) were fixed and stained with X-gal. Tumors (arrow in C) are found in 15-20% of mutant mice between 10-25 weeks of age; these tumors express high levels of the mutant patched allele, which carries the ß-galactosidase gene. At earlier ages, >50% of patched mutant mice have ectopic ß-galactosidase-expressing cells (arrowheads in B). Background X-gal staining was not detected in the cerebellum of adult wild-type mice (A).