This allele was derived from mating Tg(GFAP-lacZ-TAg121)92Tvd animals to mice carrying a cre transgene (Tg(ACTB-cre)2Mrt). As a result the intervening floxed lacZ cassette was removed, leaving a single loxP site and allowing transcription from the mutant SV40 T antigen sequence. The mutant SV40 T antigen 121 contains a deletion for the small t antigen-specific splice signal and a deletion that results in truncation of the large T antigen at amino acid 121. The mutant antigen binds RB1/RBL1/RBL2 but not TRP53. Immunohistochemical analysis of progeny showed diffuse expression of this transgene throughout the brain.