The thrombospondins (TSPs) are a family of multimodular proteins that bind to the extracellular matrix with strong affinity. Of the five members of the TSP family, TSP1 and TSP2 are the only ones that inhibit endothelial cell migration in vitro and neoangiogenesis in vivo. This angiogenesis-inhibiting effect is mediated by interaction of a short sequence in type I modules with the membrane receptor CD36. TSP1 and TSP2 gene knockout experiments in mice showed increased blood vessel density in TSP2 -/- but no such alteration in TSP1 -/- animals. Loss of TSP1 gene expression was correlated with acquisition of an angiogenic phenotype in several models of human malignant tumors. Taken in concert, these findings suggest that TSP1 and, to a lesser extent, TSP2 may have therapeutic potential as angiogenesis-inhibiting factors.