Scientists in three countries have discovered a gene needed to start puberty.

American researchers zeroed in on it by studying families in which cousins had married each other. Researchers in France and England identified the gene simultaneously.

The gene, called GPR54, is not on the X or Y sex chromosome. Instead, it is on one of the ordinary paired sets of chromosomes, No. 19. That means one good copy is enough to start puberty. But two defective copies -- one from each parent -- can keep the body from maturing, a condition that usually can be treated with hormones.

The American and British researchers reported their findings in Thursday's issue of the New England Journal of Medicine. The French group published its own findings in the Proceedings of the National Academy of Sciences.

"Findings from this study may lead not only to more effective treatments for individuals who fail to enter puberty normally, but may provide insight into the causes of other reproductive disorders as well," said Dr. Duane Alexander, director of the National Institute of Child Health and Human Development, which funded the American study.

The researchers note that GPR54 is probably not the only gene whose action is required for normal puberty but rather one of an ensemble of genes in charge of the complex process.

"Our challenge moving forward will be to determine how directly this receptor affects the reproductive hormone cascade," said researcher Dr. Stephanie Seminara, of the Reproductive Endocrine Unit at Massachusetts General Hospital.