Platelet-derived endothelial cell growth factor (ECGF1) is an angiogenic factor which promotes angiogenesis in vivo and stimulates the in vitro growth of a variety of endothelial cells. ECGF1 has a highly restricted target cell specificity acting only on endothelial cells. Because it limits glial cell proliferation, ECGF1 is also known as thymidine phosphorylase and as gliostatin. The ECGF1 gene contains 10 exons spanning more than 4.3 kb. Thymidine phosphorylase activity of ECGF1 in leukocytes from mitochondrial neurogastrointestinal encephalomyopathy (MNGIE) patients was less than 5 percent of controls, indicating that loss-of-function mutations in thymidine phosphorylase cause MNGIE.