The ability to fertilize an egg is acquired during passage through the female reproductive tract and requires two sequential, time-sensitive processes to provide the sperm with the physical and biochemical properties necessary for fertilization. The process of capacitation is the process that changes the physiology of each sperm by changing the membrane of the head to help it penetrate the hard outer layer of the egg and the chemistry in the tail to create greater motility, the ability to move and swim. The acrosome reaction (AR) is a chemical action that involves the release of enzymes in the head of the spermatozoon, which further accelerate penetration of the coat. Both processes are important for successful fertilization of the egg, and AR is time dependent: it cannot happen too early or too late. Premature AR is associated with idiopathic (spontaneous) male infertility. Researchers at the University of California, San Diego School of Medicine explain how GIV/Girdin, a signaling molecule, plays a critical role in male fertility by controlling motility and AR to promote sperm motility, survival and fertilization success. HIV is essential for male fertility, and low levels of HIV transcripts in men are consistently associated with infertility.