Scientists have discovered a gene which is crucial to male fertility.
A lack of sperm cells in mice without the gene
Researchers made the discovery by chance, while looking into the genetic causes of heart disease.
They genetically engineered mice so they did not have the Fkbp6 gene.
Researchers found no link between the gene and heart disease, but they did find that all the sperm cells in the male mice died, meaning they were completely infertile.
The females' oocyte cells - cells in the early stages of egg development - were unaffected.
The Canadian team which carried out the research say the discovery of the gene could provide a focus for the development of a male contraceptive pill.
It could also lead to the development of a test for the 15% of couples around the world who are infertile to see if a lack of the Fkbp6 gene is the cause.
When the researchers studied the mouse sperm cells and tissue, they found they completely lacked spermatids, the male germ cells which eventually develop into spermatozoa.
The lack of the Fkbp6 gene was identified as the cause of their infertility.
The research is published in the magazine Science.
Normal sperm cell development in male mice
Dr Josef Penninger, professor of medical biophysics and immunology at the University of Toronto who led the research, said: "That this gene would control male fertility was completely unexpected.
"We found no link between this gene and heart disease but we did find that our male mice were unable to breed.
"When we investigated further, we found that the size of the testes of our mice were massively reduced and that they produced no sperm cells.
"Fkbp6 only acts in sperm cells and we found no other defects in our mice besides sex-specific male infertility.
"While our male mice showed normal sexual behaviour and had normal levels of sex hormones, they completely lacked sperm cells.
"So it's possible that Fkbp6 might be the perfect target for the development of a male birth control pill."
Japanese researchers had seen a similar mutation in rats with aspermia, a lack of sperm.
The Canadian team found the Fkbp6 gene is also crucial to the process of matching up maternal and paternal chromosomes.
Fkbp6 is part of the protein complex that binds maternal and paternal chromosomes in oocytes and sperm cells.
Normally, each chromosome matches up with a chromosome of the same structure, position and origin.
In the mice which did not have Fkbp6, the chromosomes were unable to find their correct partners and, instead, paired with non-corresponding chromosomes.
This kind of defect can lead to a condition called aneuploidys, where there are too many or too few chromosomes. It is a leading cause of spontaneous miscarriages in humans and a trait in many human cancer cells.
The researchers say proteins produced by the Fkbp6 gene, and other genes in the same family, are essential for chromosome pairing to work.