The Thp deletion on mouse chromosome 17 is lethal when inherited from the mother, because it deletes the T-associated maternal effect (Tme) locus, the paternal copy of which is inactivated by genomic imprinting. We have found a paternally nonimprinted Tme variant in crosses of Thp females with Mus m. musculus males. The data are consistent with the existence of a single Tme-unlinked gene, Imprintor-1 (Imp-1), with two alleles, one of which only causes imprinting at the Tme locus. Imp-1 is unlinked to the gene for cation-dependent Man-6-P receptor and acts prezygotically. Although Tme and Igf2r were thought to be identical, they show different patterns of imprinting in interspecies hybrids. The apparent nonequivalence of the Igf2r gene and Tme results in occurrence of viable mice lacking an active Igf2r gene. These mice are bigger at birth than their normal littermates, in accord with the proposed function of the IGF-II/Man-6-P receptor.