At the Ludwig Institute for Cancer Research, a hybrid construct containing H-2Kb(MHC Class I antigen) 5' promotor sequences fused to the early region of the SV40 mutant tsA58, which encodes both the large and small SV40 tumor antigens, was microinjected into fertilized oocytes from CBA/Ca x C57BL/10 F1 mice. Following reimplantation, 88 mice were born, of which 34 were transgenic and carried one to five copies of the gene. RNA from a variety of tissues from one nontransgenic and three transgenic animals was analyzed by Northern blot analysis using a SV40 early region-specific probe. RNA extracted from tissues of transgenic mice contained varying amounts of a 2.5kb RNA species, while no tsA58TAg RNA was detected in tissues of the nontransgenic mouse; thymus and liver showed the highest level of expression, while brain showed the lowest. Distribution rights to Charles River in 1991.