In situ hybridization showing localization of transgene expression by autoradiography and dark-field microscopy in nondrug-treated mice. a, d, P1 transgenic and nontransgenic cerebellum, respectively. b, e, Adult transgenic and nontransgenic cerebellum, respectively. c, f, Adult transgenic and nontransgenic testes, respectively. Note that there is no specific labeling in the external granule layer in the P1 transgenic cerebellum (a) nor in the internal granule cell layer of the adult transgenic cerebellum (b), as compared with the background level seen in nontransgenic controls (d, e). Specific labeling is highest in the Purkinje cell layer, which contains the cell bodies of Bergmann glia but is also apparent in white matter. Expression in postmitotic spermatids of the testis (c) has been observed previously in other TK transgenic models.p, Pial surface; arrow points to Purkinje cell layer in P1 and adult cerebella.