HONG KONG (Reuters) - Researchers in Japan have identified a gene that appears to determine cyclical hair loss in mice and believe it may also be responsible for hair loss, or alopecia, in people.

In a report published in the Proceedings of the National Academy of Sciences, the scientists described how they generated a line of mice that were lacking in the Sox21 gene.

"The mice started to lose their fur from postnatal day 11, beginning at the head and progressing toward the tail region of the back," they wrote.

"Between day 20 and day 25, these mice eventually lost all of their body hair, including the whiskers. Intriguingly, new hair regrowth was initiated a few days later but was followed by renewed hair loss."

The cyclical alopecia continued for more than two years and the researchers observed 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 gene is likely involved with the differentiation of stem cells that form the outer layer of the hair shaft," wrote the researchers, led by Yumiko Saga of the Division of Mammalian Development at the National Institute of Genetics in Mishima.

The scientists went on to examine human skin samples, where they found evidence of this same gene.

"We confirmed that Sox21 is also expressed in the hair shaft cuticle in humans ... These results indicate that the Sox21 gene could be responsible for some hair loss conditions in humans," the authors concluded.

(Reporting by Tan Ee Lyn; Editing by Alex Richardson)