IL-6 induces MAP kinase–dependent Id1 expression in primitive progenitors. Sorted LSK fraction from Id1^GFP/GFP mice was cultured in serum-free, stromal cell–free conditions as described^19,38  and with added carrier (media) or IL-6 or IL-3 as indicated. Id1 expression was analyzed by GFP expression level using flow cytometry. (A) Id1 expression is shown 24 hours after treatment with or without IL-6 (10 ng/mL) or IL-3 (10 ng/mL). The results are representative of 3 separate experiments. (B) The relative ratio of GFP⁺ to GFP⁻ cells is shown. *P < .005 compared with the control. (C) Sorted cells were placed in serum-free stroma cell–free system with IL-6 10 ng/mL and treated with carrier (DMSO) or MAPK inhibitor (U0126) 10 μM. Id1 expression was determined by flow cytometry after 24 hours of culture. The data were obtained from 3 different experiments and are shown as an average (mean ± SD). *P < .005 compared with the control. (D) The relative expression of Id1 mRNA is shown. The purified LSK cells were cultured in serum-free, stroma-cell free system with IL-6 (10 ng/mL) and treated with control (DMSO) or MAPK inhibitor (U0126) 10 μM, as in panel C. The total RNA was extracted from each sample after 12 hours of incubation, and real-time PCR was conducted. Id1 expression was normalized by 18S rRNA expression.