Hair - the long and the short of it.

When Rapunzel let down her hair for the prince to climb up, she had no clue that a genetic defect probably lay at the root of her golden locks. But then, neither did anyone else until an experiment in developmental biology went awry.

Researchers studying a chemical messenger called fibroblast growth factor 5 had created mice that lacked the gene for this messenger to see how its loss would affect embryonic development. To their surprise, the newborn mice looked and acted normal, says Gail L. Martin, a developmental biologist at the University of California, San Francisco. But a few weeks later, she and her colleagues noticed that the mice with the missing growth factor looked a bit shaggy. "When [the factor] is missing, hair grows very, very long," Martin reported this month at the annual meeting of the Society for Cell Biology, held in San Francisco.

Normally, hair grows in cycles. First, a hair follicle develops. Deep inside it lies a bud of mesodermal tissue. That bud then divides and sprouts as a hair, but eventually it stops growing. The follicle becomes quiescent, and the hair falls out. The cycle then begins again.

In the experimental mice, the follicles appear normal and the hair grows at the usual rate. However, these follicles -- unlike normal ones -- don't make the growth factor, which, contrary to its name, appears to limit hair growth, Martin says. As a result, hair grows for a longer time during each cycle.

Her group's genetic analyses indicate that a known gene called angora is actually a variant of the gene for this growth factor. She expects people, too, may have the angora variant and seeks people with very long hair for testing.

According to Martin, fibroblast growth factor 5 is the first, but probably not the only, chemical signal discovered for the hair cycle. "There's obviously a backup signal, because the hair doesn't grow forever," she notes.