Clathrin-mediated endocytosis is a central and well-studied trafficking process 
in eukaryotic cells. How this process is initiated is likely to be a critical 
point in regulating endocytic activity spatially and temporally, but the 
underlying mechanisms are poorly understood. During the early stages of 
endocytosis three components-adaptor and accessory proteins, cargo, and 
lipids-come together at the plasma membrane to begin the formation of 
clathrin-coated vesicles. Although different models have been proposed, there is 
still no clear picture of how these three components cooperate to initiate 
endocytosis, which may indicate that there is some flexibility underlying this 
important event.