In the presence of inorganic orthophosphate, the platelet-derived endothelial growth factor (PD-ECGF) thymidine phosphorylase (TP) (gliostatin) catalyses the reversible phospholytic cleavage of thymidine and deoxyuridine to their corresponding bases and 2-deoxyribose-1-phosphate. It is both chemotactic and mitogenic for endothelial cells and a non-heparin binding angiogenic factor present in platelets. It is also involved in transformation of fluoropyrimidines, cytotoxic agents used in the treatment of a variety of malignancies, into active cytotoxic metabolites.