Before a sperm can successfully fertilize an egg, it must bind to the egg's outer layer, the zona pellucida. This attachment is probably highly specific, as sperm will only recognize eggs from the same species. So what controls sperm–egg attachment? Writing in Cell (114, 405–417; 2003), Michael A. Ensslin and Barry D. Shur take a step towards answering this question — and their findings could offer clues to why some males with apparently 'normal' sperm are infertile.

In a hunt for egg-binding molecules, the authors looked at a protein from mouse testes, which they named SED1. This protein was a likely suspect because it was similar to a protein from boar sperm that had previously been shown to interact with components of the zona-pellucida. They found that SED1 was located on the surface of mouse sperm — a good start. But more interesting was the finding that antibodies against SED1 blocked sperm–egg binding.

Of course, what happens in a test tube is not always important in animals, so the authors looked next at the effect of eliminating SED1 in mice. They found that genetically engineered mice that lacked SED1 were much less fertile than normal mice. And although the sperm from these mice moved normally and were produced in the usual quantities, they failed to bind to eggs — proof that SED1 is indeed needed for sperm–egg binding.