Scientists know surprisingly little about the molecular mechanics of how sperm fertilize eggs; only three proteins central to the process have ever been identified, all in mammals. A study in Science reports that a tiny protein on the surface of zebrafish eggs acts as a species-specific gatekeeper for sperm, giving fertilization the green light only if the sperm is also from a zebrafish (2018, DOI: 10.1126/science.aat7113). Researchers in Andrea Pauli’s lab at the Research Institute of Molecular Pathology stumbled on the gene for this 80-amino-acid protein in a poorly understood region of the zebrafish genome. Zebrafish engineered to lack the gene produced eggs that couldn’t be fertilized by zebrafish sperm. The team used the gene-editing tool CRISPR to engineer zebrafish so their eggs expressed a homolog from a different fish called a medaka. Zebrafish sperm couldn’t fertilize the egg, but medaka sperm could. The researchers called the zebrafish protein Bouncer, a reference to a security guard at a bar. Such species specificity is crucial for animals like fish and frogs that release their sperm and eggs into the water for fertilization. Pauli’s team is now studying whether Bouncer’s mammalian homolog—a protein called SPACA4, which is expressed in sperm rather than eggs—is also involved in fertilization.