ID1 and neoplasm: While this issue is not resolved fully by these studies, our work supports the idea that Id1-expressing BM-derived EPCs are a critical component of tumor neovascularization: Id1 is required for normal tumor angiogenesis, Id1 expression in HSCs is required for the generation of progenitor cells with the ability to become incorporated into blood vessels, Id1 is only expressed in tumor endothelial cells and not macrophages or pericytes [6], and Id1 loss does not affect the levels of monocytes, macrophages or neutrophils in the peripheral blood or at the site of the tumor (Figure 5).