We have investigated the role of the mammalian Son of sevenless 1 (Sos1) protein in growth factor signaling in vivo by generating mice and cell lines that lacked the Sos1 protein. Homozygous null embryos were smaller than normal, died mid-gestation with cardiovascular and yolk sac defects, and their fibroblasts showed reduced mitogen-activated protein kinase activation in response to epidermal growth factor (EGF). An intercross of mice mutant for Sos1 and the EGF receptor (EGFR) demonstrated that a heterozygous mutation in Sos1 dominantly enhanced the phenotype of a weak allele of the EGFR allele (wa-2). These animals had distinctive eye defects that closely resembled those seen in mice that were null for the EGFR or its ligand, TGF alpha. Our findings provide the first demonstration of a functional requirement for Sos1 in growth factor signaling in vivo. They also show that the genetic test of enhancement of weak receptor allele by heterozygous mutation in one component represents a powerful tool for analyzing the ras pathway in mammals.