During the formation of sperm, protamine binds to the phosphate backbone of DNA using the arginine-rich domain as an anchor. DNA is then folded into a toroid, an O-shaped structure, although the mechanism is not known. A sperm cell can contain up to 50,000 toroid-shaped structures in its nucleus with each toroid containing about 50 kilobases.[3] Before the toroid is formed, histones are removed from the DNA by transition nuclear proteins, so that protamine can condense it.