Scientists in Hong Kong and China have identified for the first time a protein in sperm from humans and from mice that could be responsible for many unexplained cases of male infertility.

Defective versions of the protein, called epithelial ion channel, have previously been reported to be responsible for female infertility.

Writing in the latest issue of the Proceedings of National Academy of Sciences journal, the researchers said they detected the protein in sperm samples from mice and human subjects.

“(The protein) is involved in the transport of bicarbonate, which is required for sperm activation in order to fertilize the egg. If you have a defect in this (protein), then fertilization capacity of the sperm will be impaired or reduced,” Chan Hsiao Chang, physiology professor at the Chinese University in Hong Kong, said in a telephone interview on Thursday.

Experiments showed that sperm taken from mutant mice with defective versions of the protein had far lower fertility than sperm taken from normal mice, the researchers said.

The discovery would help doctors more accurately diagnose and explain many cases of male infertility that have so far gone unexplained.

“For many people, they are infertile, but they don’t know why, so diagnosis would be the immediate advantage,” Chan said.

Between 8 percent and 12 percent of couples with women of childbearing age - or between 50 and 80 million people - are infertile globally, according to the World Health Organisation.

Half of infertile couples fail to reproduce because of problems with male fertility.