Abelson-induced transformation is dependent on Stat5a/b in vitro. (A) Ab-MuLV–induced colony formation of Stat5a/b^+/+, Stat5a/b^null/+, and Stat5a/b^null/null fetal liver cells in methylcellulose. Single-colony pictures of each phenotype are depicted in the bottom panels. Stat5a/b^null/null cells showed no ability to form growth factor–independent colonies. (B) Summary of data obtained from Ab-MuLV–induced colony formation assays represent means ± SEM of 4 embryos per genotype (each performed in triplicates). (C) Surface expression of B-lineage markers was verified by flow cytometric analysis (right; data of one representative CD19⁺ CD43⁺ colony is shown). (D) [³H]thymidine incorporation of fetal liver–derived Stat5a/b^+/+ and Stat5a/b^null/+ Ab-MuLV–transformed cell lines. Stat5a/b-deficient fetal livers did not give rise to stable transformed cell lines. Data represent means ± SEM of 2 cell lines per genotype. cpm indicates counts per minute. (E) Ab-MuLV–induced colony formation of Stat5a/b^+/+ (n = 2), Stat5a/b^null/+ (n = 2), and Stat5a/b^null/null (n = 5; pooled) survivor bone marrow cells in methylcellulose. Stat5a/b^null/null survivor cells showed no ability to form growth factor–free colonies. Experiment was performed in triplicates. Asterisks denote significant differences as determined by a one-way ANOVA followed by a Tukey test (A,C) or a paired t test (B).