DT40GR cells stably transfected with vector alone (control), Flag-tagged Gab2 (wtGab2), the Y614F mutant (Gab2 Y614F), or the Y643F mutant (Gab2 Y643F), were cultured in triplicate in basal medium, or basal medium plus either G-CSF (+ G-CSF) or added serum (+ serum). The cells were pulsed with tritiated thymidine to assess new DNA synthesis; incorporation of radioactivity was determined by scintillation counting. Results are expressed as mean ± standard deviation. Compared to cells transfected with wild-type Gab2, G-CSF stimulated proliferation of cells expressing Gab2 Y643F was markedly inhibited (p < 0.01). Cells expressing Gab2 Y614F showed a smaller decrease in G-CSF-stimulated tritiated thymidine incorporation (p = 0.04). The presence of wild-type Gab2, or either of the Gab2 mutants, had no significant effect on serum-stimulated proliferation.