This targeted mutation in the Sfpi1 locus causes death of mutant homozygotes in late gestation. These embryos showed a multilineage defect in generation of progenitors for B and T lymphocytes, for monocytes, and for granulocytes. Megakaryocytes and erythroid progenitor cells were produced in normal numbers, though erythroblast maturation was impaired in some mice. The affected lineages require a function acting at the level of a common progenitor cell (J:20348); perhaps the induction of colony stimulating factor genes may represent at least part of the effect.