Mice and humans share the same gene for growing and shedding hair. But when it is faulty, hair doesn't grow back normally and over time people go bald, to varying degrees.

A healthy gene fights a natural protein called Wise, which if left to accumulate can lead to baldness. But researchers at the Kennedy Krieger Research Institute in America and Johns Hopkins University in the UK discovered that by introducing a normal gene in bald mice they were able to restart the process, resulting in successful fur growth.

They said the findings shed light on the hair growth process, much of which was not understood. But they added that more research was needed as there were other components involved in hair growth.

A Variety of Causes

Lead researcher Catherine Thompson said: "Hair is maintained through a cyclic process that includes periodic regeneration of hair follicles in a stem cell-dependent manner.

"The hair cycle consists of three defined stages -- growth, followed by regression and rest.

"Disruption of hairless gene function causes a complex skin phenotype that includes specific defect in hair follicle regeneration in both humans and mice."

Barry Stevens, general secretary of the Trichological Society, which represents hair professionals, said: "The thinking in the US is that there is some sort of genetic solution to hair loss.

"And I think eventually they will be proved right, but there is still work to do."

However, Mr. Stevens pointed out that a faulty hairless gene is just one of a variety of causes for baldness.

He added: "I think it is like a giant jigsaw and pieces of research such as this will play a role.

"But the truth is we just don't know when the solution will be found, you cannot prophesize on these things."

Two in Every Three Men

Two in every three men suffer some form of baldness by the time they are 50.

Women are affected too, although the condition alopecia is rare.

In fact, eunuchs are the only people not to suffer from baldness.

A study in 1960 of 21 men who had been castrated found that baldness was even reversed after the operation. But that is a cure many, if not all, baldies would seriously balk at.

Baldness is not only a human trait.

Chimpanzees, stump-tailed macaques, and South American nakari also develop baldness.