An application of topical estrogen to the penis over a two-week period could inhibit the spread of HIV by thickening the natural keratin layer on the skin. The epithelium of the human penis is richly supplied with estrogen receptors, explains Andrew Pask of the University of Melbourne, suggesting it could respond to topical estrogen, but unlike the vagina has only a very thin protective keratin layer. Pask suggests that in countries and cultures where circumcision is uncommon thickening this keratin layer could reduce the ability of HIV to penetrate the epidermis of the foreskin. HIV is one of the greatest health crises the world has ever seen, and affects over 40 million people worldwide. "We now have found a new avenue to possibly prevent HIV infection of the penis," says Pask.