HONG KONG (AFP) - Hong Kong scientists said Friday they have discovered a molecule that binds human sperm to an egg, in a breakthrough which offers new hope for infertile couples. The study by researchers at the University of Hong Kong (HKU) found a key molecule on the coating of the human egg, called sialyl-LewisX (SLeX), which acts as a binding agent to help the sperm and egg stick together. This research provides an enlightening answer to a basic important question and human fertilisation how does a sperm bind to an egg? William Yeung, one of the researchers, told AFP. But this is only a first step that will lead to more discoveries, said Yeung, who is also a professor at HKUs department of obstetrics and gynaecology.