A chemical called vascular endothelial growth factor (VEGF) has been implicated as a potential cause of the abnormalities seen in DME. It has been shown to modulate the growth and pattern of blood vessels, the tone and permeability of their walls, and to cause white blood cells to be drawn to the inner walls of blood vessels. [5, 6] The actual roles played by VEGF are in part dependent on the needs of the tissues in which it is acting. [5] Studies of diseases of the eye that involve the abnormal development of new blood vessels, such as neovascular age-related macular degeneration (AMD) and proliferative diabetic retinopathy (PDR), have demonstrated a central role for VEGF. It has also been implicated in the development of some of the problems associated with central retinal vein occlusion (CRVO) and DME. VEGF exists in a variety of subtypes, called "isoforms", each described by the number of amino acids (the building blocks of proteins) that it contains. The different VEGF isoforms have distinct activities at different sites in the body and its various tissues. [7] This may in part help to explain how VEGF is able to elicit such a variety of functions within blood vessels and beyond. [5]