British scientists have discovered an elusive protein that forms an essential part of human fertilisation.
The molecule, named Juno after the Roman goddess of fertility, allows eggs and sperm to join together.
It may now be used by scientists to improve fertility treatments and develop new contraceptives.
"We have solved a long-standing mystery in biology by identifying the molecules displayed on all sperm and egg that must bind each other at the moment we were conceived," said lead researcher Dr Gavin Wright, from the Wellcome Trust Sanger Institute in Cambridgeshire.
"Without this essential interaction, fertilisation just cannot happen," he added, writing in the journal Nature.
Fertility expert Dr Allan Pacey, from University of Sheffield, described the finding as "very exciting".