During fertilization, a sperm must first fuse with the plasma membrane and then penetrate the female egg cell in order to fertilize it. Fusing to the egg cell usually causes little problem, whereas penetrating through the egg's hard shell or extracellular matrix can present more of a problem to the sperm. Therefore, sperm cells go through a process known as the acrosome reaction which is the reaction that occurs in the acrosome of the sperm as it approaches the egg. The acrosome is a caplike structure over the anterior half of the sperm's head.

In some echinoderms, including starfish and sea urchins, a major portion of the exposed acrosomal content contains a protein that temporarily holds the sperm on the egg's surface

Bindin is an insoluble protein coating the sperm acrosome process and mediating the adhesion of sperm to sea urchin eggs.