Japanese scientists have identified a gene that may explain why some people lose their hair.

Researchers at the Division of Mammalian Development at the National Institute of Genetics in Mishima, say the gene appears to determine cyclical hair loss in mice and they suspect it may also be the culprit in hair loss, or alopecia, in people.

The researchers say that hair is maintained through a cyclic process that includes periodic regeneration of hair follicles in a stem cell-dependent manner, but little is known about the cellular and molecular mechanisms that regulate the layered differentiation of the hair follicle.

The scientists generated a line of mice that were lacking in the Sox21 gene which encodes a HMG-box protein - the mice developed progressive fur loss - and become completely nude in appearance, by the time they were 20 to 25 days old; their hair started to regrow a few days later but was followed by renewed hair loss.

The research team say the Sox21 is expressed in the cuticle layer and the progenitor cells of the hair shaft in both mice and humans and the lack of this gene results in a loss of the interlocking structures required for anchoring the hair shaft in the hair follicle, and the keratins and keratin binding proteins in the hair shaft cuticle are also affected.

The mice started to lose their fur by the time they were 11 days old and the hair loss which began at their heads, progressed through to their tails and included their whiskers.

This cycle of alopecia continued for more than two years and the researchers found that the mutant mice had enlarged oil-secreting sebaceous glands around the hair follicle and a thickened layer of skin cells during periods of hair loss.

The researchers, led by Yumiko Saga say that the Sox21 gene is a master regulator of hair shaft cuticle differentiation and is probably involved with the stem cells that form the outer layer of the hair shaft - when they examined human skin samples, they found evidence of the same gene.

The researchers say this indicates that the Sox21 gene could be responsible for some hair loss conditions in humans and human hair disorders.

The research is published in the Proceedings of the National Academy of Sciences.