RAP-011 expands erythroid cells in zebrafish embryos. (A-B) Expansion of ICM observed in RAP-011–treated embryos but not in IgG control. The arrow points to the ICM region. (C) Western blot of EGFP from LCR2:EGFP embryos treated with IgG or RAP-011. (D) In situ hybridization for gata1 expression in IgG or RAP-011–treated embryos at 24 hpf. (E) In situ hybridization for PCNA expression in IgG or RAP-011–treated embryos at 24 hpf. The arrows in (D-E) point to the expanded ICM region. Observed phenotype (D): WT + IgG = 30/30 WT + RAP-011 = 26/30; (E) WT + IgG = 29/30 WT + RAP-011 = 27/29.