(PLEASE KEEP IN WORD, DO NOT PDF) This project is rooted in the clinical observation that the blood diseases that affect infants, children, and adults differ. We hypothesize that that is due to developmental and age-related intrinsic differences in the fundamental properties of hematopoietic stem and progenitor cells (HSPCs). Our research group is focused on understanding how temporal differences in HSPCs impact manifestation of blood diseases. The preliminary data supporting this proposal expand our prior studies on the role of the heterochronic Lin28b/let-7 axis in defining the maturation states of definitive HSPCs. Lin28b/let-7 acts as a molecular switch whereby Lin28b is expressed in the fetal state to implement hallmarks of juvenile hematopoiesis such as fetal globin expression, erythroid-biased output, and innate-like lymphocytes. Developmental downregulation of Lin28b releases let-7 microRNAs that target transcripts to establish mature adult myeloid-biased hematopoiesis. We have found that the Polycomb repressor complex 1 (PRC1) component Cbx2 is a target of let-7 microRNAs in the hematopoietic system and that PRC1 controls the expression of master hematopoietic transcription factors (TFs) such as Erg. On the basis of this finding, we hypothesize that histone H2A lysine 119 monoubiquitylation (H2AK119Ub) by PRC1 downstream of Lin28b/let-7 regulates the TF networks that control HSPC self-renewal and differentiation. This proposal aims to understand 1) how Lin28b is normally developmentally downregulated and how it may be aberrantly expressed in blood diseases with oncofetal gene expression such as myelodysplastic syndrome; and 2) how Erg, as a master hematopoietic TF, effects Lin28b/let-7/Cbx2’s control of hematopoietic maturation. Our experience in normal hematopoietic maturation, modeling of blood diseases, and the collaborations that we have established to complete this proposed research position us to answer these key questions, which we believe have immediate applications to better understanding age-biased blood diseases.