Expression of an activator isoform of STAT5 inhibits primitive erythropoiesis in the VBI. Embryos were injected at the 4-cell stage into the 2 presumptive “ventral” blastomeres, to target RNA to the presumptive posterior VBI. Shown are representative embryos processed by in situ hybridization at stage 35 with probes for embryonic α-globin (left panels, A-E) or SCL (right panels, F-J). Embryos were injected with RNA encoding lacZ as a control (A, F), STAT5 (B, G), a mutated form of STAT5 that should be inactive (C, H), a constitutively activated isoform STAT5-VP16 (D, I), or a repressor isoform STAT5-EnR (E, J). The region of the pVBI that is inhibited for erythropoiesis by STAT5-VP16 is indicated in panel D. Views are ventral, with anterior to the left. The data are consistent with a normal function for STAT5 as a repressor of genes that inhibit globin expression. While the transcript patterns can vary somewhat from embryo to embryo, the phenotypes seen were reproducible in at least 3 independent experiments. The embryos shown here are derived from one experiment, where the expression pattern shown is most representative from a batch of (n) embryos: A, 12 of 12; B, 11 of 11; C, 10 of 15; D, 7 of 9; E, 13 of 13; F, 17 of 20; G, 10 of 13; H, 8 of 13; I, 9 of 12; J, 10 of 14.