Ab-MuLV infection induces outgrowth of B-cell precursors in Jak1(–/–)fetal livers. (A) Fetal livers were split in half, with one half mock infected and the other half infected with Ab-MuLV, and transferred onto IL-7–producing feeder cultures (T220-29 cells). Six days thereafter, the cells were analyzed for the presence of CD19/CD43 B cells by FACS analysis. (B) Immunoprecipitation of Jak1 in wild-type cells. Three individual Jak1(+/–) cell lines were analyzed for the expression and tyrosine phosphorylation (P-Tyr) of Jak1. C indicates v-abl–transformed cells stimulated with 10 ng/mL interleukin-7. (C) Immunoprecipitation of Jak2. Two Jak1(–/–) and 2 Jak1(+/–) cell lines were used to investigate the expression and tyrosine phosphorylation of Jak2. As a control (indicated by C), v-abl–transformed Jak1(+/–) cells were stimulated with 10 ng/mL IFN-γ. (D) Immunoprecipitation of Jak3. Two Jak1(–/–) and 2 Jak1(+/–) cell lines were used to investigate the expression and tyrosine phosphorylation of Jak3. As a control (indicated by C), v-abl–transformed Jak1(+/–) cells were stimulated with 10 ng/mL interleukin-7. The upper strongly tyrosine-phosphorylated band corresponds to v-abl that associates with Jak3 upon activation.