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Studies in invertebrates have led to the identification of a number of genes that regulate lifespan, some of which encode components of the insulin or insulin-like signalling pathways^,,. Examples include the related tyrosine kinase receptors InR (Drosophila melanogaster) and DAF-2 (Caenorhabditis elegans) that are homologues of the mammalian insulin-like growth factor type 1 receptor (IGF-1R). To investigate whether IGF-1R also controls longevity in mammals, we inactivated the IGF-1R gene in mice (Igf1r). Here, using heterozygous knockout mice because null mutants are not viable, we report that Igf1r^+/- mice live on average 26% longer than their wild-type littermates (P < 0.02). Female Igf1r^+/- mice live 33% longer than wild-type females (P < 0.001), whereas the equivalent male mice show an increase in lifespan of 16%, which is not statistically significant. Long-lived Igf1r^+/- mice do not develop dwarfism, their …