SOCS3^ΔSB/ΔSB bone marrow cells are hyperresponsive to G-CSF. (A) Bone marrow cells (25 000) from WT, SOCS3^ΔSB/ΔSB, or SOCS3^ΔVAV/− mice were plated in semisolid agar cultures with cytokines as indicated and colonies were counted after 7 days. Results represent the mean (± SD) from at least 3 mice of each genotype. (B) To ascertain colony size, cultures were plucked, resuspended, and counted. Results represent the mean (± SD) for at least 3 mice of each genotype. (C) To assess synergy between SCF and other cytokines, 25 000 bone marrow cells from SOCS3^+/+, SOCS3^ΔSB/ΔSB, or SOCS3^ΔVAV/− mice were plated in semisolid agar cultures stimulated by SCF and/or additional cytokines as indicated and colonies were counted after 7 days. Results for each cytokine are represented as groups containing WT, SOCS3^ΔSB/ΔSB, and SOCS3^ΔVAV/− data for the single cytokine, SCF, and the combination of the 2. Results represent the mean (± SD) from at least 3 mice of each genotype. Synergy between SCF and IL-6 was not detected in cultures of SOCS3^ΔVAV/− bone marrow cells. (D) Average colony size. Results represent the mean (± SD) for at least 3 mice for each genotype. Again, synergy between SCF and IL-6 was not observed in cultures of SOCS3^ΔVAV/− cells. (E-F) Bone marrow cells were stimulated with G-CSF (E) or IL-3 (F) and proliferative activity was assessed after 48 hours by ³[H]-thymidine incorporation. Results represent the mean (± SD) for 2 mice per genotype; analysis of each mouse included 6 replicates (*P < .05; **P < .01).