This is one of several targeted null mutations of Rb1 that have been created. Results appear to be similar for all the mutations (J:2498, J:2511, J:2516). Heterozygotes for the mutations show no predisposition to retinoblastoma. Homozygotes die in utero with neuronal and hematopoietic system abnormalities. Transfer of a human RB1 mini-transgene into the mutant mice corrects the defects (J:2516). On the other hand, transfer of the human gene into mice with a normal Rb1 genotype, causing overexpression of the gene product, produces mice dwarfed in proportion to the number of extra RB1 copies they carry (J:15042). Homozygous Rb^1tm1Tyj mutant mice given a transgene producing low levels of Rb1 product survive to birth, but die at that stage due to failure of myogenesis. Myoblasts undergo massive apoptosis, and surviving cells do not undergo terminal differentiation (J:37145).