While a role for Id1 in the non-BM derived endothelial cell component of some tumors has been established [6], in other settings it is clear that the phenotype observed in the Id1-/- mice is due to a failure in the BM compartment since normal tumor angiogenesis can be completely rescued by transplantation with wild type BM [5], [6], [10]. The gene discussed is ID1; the disease is neoplasm.