Inhibition of let-7 in adult myeloerythropoiesis. (A) Purified WT adult BM CMPs were retrovirally transduced with an empty vector or a let-7 sponge construct and cultured in methylcellulose for 14 d, when colonies were scored by morphology. Colony distributions are presented. *, P = 0.03 by Student’s t test. n = 5 replicates for vector and sponge constructs across three independent experiments. CFU-M, CFU monocyte; CFU-G, CFU granulocyte; CFU-GM, CFU granulocyte–monocyte. (B) RNA was isolated from cultures after 14 d, and levels of mouse globin genes were measured by qPCR. *, P < 0.01 by Student’s t test. n = 3 replicates across three experiments. (C) Adult mice with the locus encoding let-7b1 and let-7c2–deleted (let-7 KO) or WT C57 littermates were euthanized, BM cells were stained with the indicated antibodies, and myeloid progenitor populations were analyzed by flow cytometry. (D) Numbers of myeloid progenitor cells are presented. *, P = 0.14; **, P = 0.009 by Student’s t test for each comparison. n = 3 WT and let-7 KO mice analyzed; results are representative of two experiments performed (a separate experiment was performed with mice of strain 129 with similar results). (E) BM from the indicated mice was isolated and stained with the indicated antibodies to evaluate granulocyte populations. (F) Numbers of Gr-1⁺/Mac-1⁺ mature granulocytes are presented. *, P = 0.09 by Student’s t test. n = 3 for each genotype; the phenotype is verified in an independent strain in an independent experiment. Error bars represent SEM.