Gene targeting was used to create a null allele at the epidermal growth factor receptor locus ( Egfr ). The phenotype was dependent on genetic background. EGFR deficiency on a CF-1 background resulted in peri-implantation death due to degeneration of the inner cell mass. On a 129/Sv background, homozygous mutants died at mid-gestation due to placental defects; on a CD-1 background, the mutants lived for up to 3 weeks and showed abnormalities in skin, kidney, brain, liver, and gastrointestinal tract. The multiple abnormalities associated with EGFR deficiency indicate that the receptor is involved in a wide range of cellular activities.