Expression of an activated FGF receptor results in increased phosphorylation of STAT5 in Xenopus embryos. Shown are representative Western blotting experiments on samples that had been immunoprecipitated with antibodies specific for the myc epitope (M) or equivalent samples that were processed similarly using an isotype-matched control antibody (C). Lysates were derived from pools of 20 stage 12 embryos that had been injected with RNA encoding LacZ as a control (lanes 1, 2), myc-tagged xSTAT5 (myc STAT5, lanes 3, 4), the constitutively activate isoform of the FGFR1 (torso) + myc-STAT5 (myc S5, lanes 5, 6), eFGF + myc STAT5 (lanes 7, 8), myc-STAT5 + the dominant-negative FGFR1 (XFD, lanes 9, 10), or the constitutively active isoform of the FGFR1 (torso) alone (lanes 11, 12). Blotted samples were probed sequentially using antibodies for total STAT5 protein (top panel), STAT5 phosphorylated specifically at tyrosine 695 (P-STAT5, middle panel), or total myc-tagged proteins (myc, bottom panel). The phospho-STAT5 levels are increased over baseline upon expression of torso (arrow in lane 6), although the baseline levels of phospho-STAT5 are not reduced in the presence of XFD.