THE receptor for insulin-like growth factor type 2, also known as the cation-independent mannose-6-phosphate receptor (Igf2/Mpr), is a multifunctional receptor thought to play a role in lysosomal targeting, cell growth and signal transduction(1-8). Igf2/Mpr has been mapped to the mouse Tme(9) locus and shown to be an imprinted gene(10), which further suggests a role in embryonic growth regulation. To define the functions of Igf2/Mpr, we have generated mice lacking this gene. We report here that maternal inheritance of an Igf2/Mpr null allele (-/+) as well as homozygosity for the inactive allele (-/-) is generally lethal at birth and mutants are about 30% larger, indicating that maternal expression of Igf2/Mpr is essential for late embryonic development and growth regulation. The phenotype is probably caused by an excess of Igf2 because the introduction of an Igf2 null allele rescued the Igf2/Mpr mutant mice. Mutant mice also have organ and skeletal abnormalities and missort mannose-6-phosphate-tagged proteins. A few (-/+) mice reactivated their paternal Igf2/Mpr allele in some tissues and survived to adults. But no (-/-) mice survived, indicating a role for the reactivated paternal allele in postnatal survival.