Conditional deletion of Spry1 leads to reticulocytosis and engagement of splenic erythropoiesis. (A) Mx1-Cre mediated deletion of Spry1. The floxed Spry1 allele used in these studies is illustrated, together with its efficient deletion after polyinosinic acid-polycytidylic acid activation of Mx1-Cre. In assessments of deletion, peripheral blood cells were analyzed by RT-PCR. (B) At steady state, Spry1 deletion selectively alters (and elevates) reticulocyte production. (C) Spry1 deletion induces splenic erythropoiesis. Flow cytometric analyses of erythroid progenitors within spleens of conditional Spry1^−/− and control mice revealed an approximate 280% increase in erythroblast frequencies resulting from Spry1 deletion. Top panels: representative primary flow data. Bottom panel: mean (± SE) erythroblast frequencies (n = 4). (D) For Spry1^−/− mice, flow analyses also identified an approximate 170% elevation of splenic stage E2 proerythroblasts.