Spry1-null hematopoietic progenitors fail to efficiently repopulate the erythron during short-term bone marrow transplantation. (A) Bone marrow cells from conditional Spry1-null mice, or wild-type controls, were transplanted to irradiated congenic recipients. At day 16, rebound hematocrits were determined. Values are mean ± SE (n = 4). Results of 2 independent experiments are illustrated. (B) Time course of erythron repopulation after short-term bone marrow transplantation of wild-type recipients as transplanted with control versus Spry1-null donor bone marrow cells. Graphed values are mean hematocrits (± SE) for n = 4 mice per time point. (C) Elevated stage E2 and E1 proerythroblast levels in mice transplanted with Spry1-null donor cells. In transplanted mice, analyses of erythroid progenitors in spleen revealed multifold increases in frequencies of Spry1- null donor-derived stage E2 proerythroblasts. Bottom panels: representative primary flow cytometry data. Bottom panel: composite data are graphed: mean ± SE (n = 4).