show Abstracthide AbstractEarly B-lymphopoiesis depends on E2A, Ebf1, Pax5 and Ikaros family members. Here, we used acute protein degradation in vivo to identify direct target genes of these transcription factors in pro-B, small pre-B and immature B-cells. E2A, Ebf1 and Pax5 predominantly function as transcriptional activators by inducing open chromatin at their target genes, have largely unique functions and maintain cell viability in vivo by activating Bcl2l1. Ikaros and Aiolos cooperatively control early B-cell development and act as dedicated repressors to suppress open chromatin at their target genes. The surrogate light-chain genes Igll1 and Vpreb1 are directly activated by Ebf1 and Pax5 in pro-B cells and directly repressed by Ikaros and Aiolos in small pre-B cells. Pax5 and E2A contribute to V(D)J recombination by activating Rag1, Rag2, Dntt, Irf4 and Irf8. Like Pax5, Ebf1 also represses the cohesin-release factor gene Wapl to mediate prolonged loop extrusion across the Igh locus. In summary, in vivo protein degradation has provided unprecedented insight into the control of early B-lymphopoiesis by five transcription factors. Overall design: ATAC-seq: Peak calling and read signal comparison between control and 5-Ph-IAA treated pro-B, pre-B and immature B cells of the same genotype. Two replicas each.