TSP1 contains three type I repeats, only the second two of which have been found to inhibit angiogenesis. The type I repeat motif is more effective than the entire protein at inhibiting angiogenesis and contains not one but two regions of activity. The amino-terminal end contains a tryptophan-rich motif that blocks fibroblast growth factor (FGF-2 or bFGF) driven angiogenesis. This region has also been found to prevent FGF-2 binding ECs, suggesting that its mechanism of action may be to sequester FGF-2. The second region of activity, the CD36 binding region of TSP1, can be found on the carboxy-terminal half of the type I repeats. It has been suggested that activating the CD36 receptor causes an increase in ECs sensitivity to apoptotic signals. Type I repeats have also been shown to bind to heparin, fibronectin, TGF-β, and others, potentially antagonizing the effects of these molecules on ECs. However, CD36 is generally considered to be the dominant inhibitory signaling receptor for TSP1, and EC expression of CD36 is restricted to microvascular ECs.