STAT5 activity is required for normal blood island development. (A) Shown are representative embryos that were either uninjected (control) or injected with both MO1 and MO2 (20 ng each) into both blastomeres at the 2-cell stage. Embryos were harvested at stage 25 and processed by whole mount in situ hybridization to detect transcript levels for embryonic α-globin, which are substantially reduced in the morphant embryos. Embryos are positioned ventral side up and anterior to the left. This experiment was repeated 3 times, and although the number of affected embryos varies, the statistics provided under “Results” are representative. (B) RNA was isolated at stage 25 from presumptive VBI regions (ventral) or whole embryos and tested for the presence of xSTAT5 RNA using semiquantitative RT-PCR. Samples for the RT reaction contained either 0.25 μg(1×) or 0.5 μg(2×) of total RNA. The xSTAT5 RNA was detected in this example using 28 cycles of PCR and still showing dependence on the amount of RNA in the original RT reaction. This experiment was repeated 3 times with similar results.