Summary of Id1^−/− hematopoietic phenotypes. Id1^−/− mice show decreased BM cellularity, which results in decreased absolute numbers of B cells and erythroid cells, while myeloid cell development is enhanced. Id1^−/− LSK and LS⁻K progenitor cell cycling is increased in vivo but not in vitro. Cytokine and chemokine production is dysregulated in stroma cultures in vitro and in vivo, suggesting that Id1 is required for proper microenvironment function. As a result of dysregulated growth factor production, Id1^−/− mice show enhanced myeloid proliferation and differentiation, increased egress of myeloid cells from the BM, and increased neutrophil and monocyte numbers in the PB.