The c-kit-encoded transmembrane tyrosine kinase receptor for stem cell factor (Kit/SCF-R) is required for normal haematopoiesis, melanogenesis and gametogenesis 1, 2, 3. However, the roles of individual Kit/SCF-R-induced signalling pathways in the control of developmental processes in the intact animal are completely unknown. To examine the function of SCF-induced phosphatidylinositol (PI) 3′-kinase activation in vivo, we employed the Cre-loxP system 4 to mutate the codon for Tyr719, the PI 3′-kinase binding site in Kit/SCF-R, to Phe in the genome of mice by homologous recombination. Homozygous (Y719F/Y719F) mutant mice are viable. The mutation completely disrupted PI 3′-kinase binding to Kit/SCF-R and reduced SCF-induced PI 3′-kinase-dependent activation of Akt by 90%. The mutation induced a gender-and tissue-specific defect. Although there are no haematopoietic or pigmentation defects in homozygous mutant mice, males are sterile due to a block in spermatogenesis, with initially decreased proliferation and subsequent extensive apoptosis occurring at the spermatogonial stem-cell level. In contrast, female homozygotes are fully fertile. This is the first report so far demonstrating the role of an individual signalling pathway downstream of Kit/SCF-R in the intact animal. It provides the first in vivo model for male sterility caused by a discrete signalling pathway defect affecting early germ cells.