Hematopoietic stem and progenitor-cell populations are preserved in fetal livers of Stat5ab^null/null mice. (A) Flow cytometric analysis of Stat5ab^null/null, Stat5ab^+/null, and Stat5ab^+/+ 14.5-dpc fetal livers for early hematopoietic cells. Top panel shows Kit and Sca-1 expression of lineage-negative fetal liver cells. Bottom panel shows CMP (FcγR^loCD34⁺), MEP (FcγR^loCD34⁻), and GMP (FcγR^hiCD34⁺) populations, with percentage of Kit⁺Lin⁻Sca⁻ falling into each progenitor population as indicated. (B) Proportion of cells that is identified as either KLS (black), progenitors (CMPs, MEPs, or GMPs, diagonal stripes), or lineage-committed (gray) hematopoietic cells within Stat5ab^+/+, Stat5ab^+/null, and Stat5ab^null/null 14.5-dpc fetal livers as determined by flow cytometric detection of cell-surface markers. (C) Cytokine-dependent colony formation of vector-transduced 14.5-dpc fetal liver cells when cells were plated in the presence of SCF, IL3, IL6, and EPO. Data shown are the mean percentage of colonies formed by Stat5ab^null/null and Stat5ab^+/null cells relative to Stat5ab^+/+ cells. Error bars represent standard deviation. Vector control Stat5ab^+/+ cells generated 688 ± 18 G418-resistant colonies per 10⁵ transduced fetal liver cells, while Stat5ab^+/null and Stat5ab^null/null cells formed 535 ± 12 and 355 ± 10 colonies. TPNeo-transduced Stat5ab^+/+, Stat5ab^+/null, and Stat5ab^null/null cells formed 337 ± 33, 559 ± 43, and 321 ± 30 G418 colonies, respectively. Transduction efficiency of Stat5ab^+/+ cells averaged 25% ± 15% and Stat5ab^null/null cells averaged 45% ± 16% for 7 independent transductions of each genotype. (KLS indicates Kit⁺Sca⁺Lin⁻; CMP, common myeloid progenitor; MEP, megakaryocyte-erythrocyte progenitor; and GMP, granulocyte-monocyte progenitor.)