The cell surface holds a wide variety of receptors, composed of proteins. These receptors control the interactions between cells and their environment. Using these receptors, cells receive signals telling them to undergo mitosis, start inner processes, activate certain genes, and even undergo apoptosis (cell death). Clearly, to make sure the cell functions properly, the type, amount, and activation of receptors on the surface must be closely regulated. Many diseases, such as AIDS, may result from cells’ inabilities to down-regulate the activity of its receptors.