ike their human hosts, hair follicles go through cycles of activity and rest. Baldness ensues when too many follicles enter the resting stage, and that happens in male pattern baldness because of an excessive quantity of dihydrotestosterone (DHT), the male hormone, within each follicle. The DHT causes the follicles to shrink and turn out ever finer, fuzzier hair.

The Food and Drug Administration has approved two basic drugs that are said to slow down hair loss and grow new hair by revitalizing weakened follicles. Minoxidil, most famously in the form of Rogaine, is a topical treatment; finasteride, marketed as Propecia, comes in pills. Studies have shown that many, but far from all, men have found the drugs helpful, but both are dose-dependent: once you stop using them, you lose any new hair, as well as the preserved old hair.

Genetic alternatives are apt to be far more effective, though they may be some years away. An early breakthrough came in 1998 with the identification of the first gene specifically associated with human hair loss. This Hairless gene, as it was appropriately named, only produces a rare type of total baldness called alopecia universalis, but its discovery enables scientists to study directly for the first time how a gene causes hair loss.

A year later, researchers found that the dormant hair follicles of mice injected with a gene called Sonic hedgehog were within a few days switched into an active, growth stage. It remains to be seen whether the gene works on humans, and whether it can be modified to avoid its sometime side effects, which include skin cancer.

The Sonic hedgehog gene was delivered to its animal subjects through an injected virus, but topical medications are easier and safer to apply. A San Diego-based company, AntiCancer Inc., has found a possible solution, using a topical mix to transport genetic material into the hair follicles of mice and induce a gene in the follicles to respond. Other scientists have used a similar system to deliver human hair follicle stem cells to sections of human scalp implanted in hairless mice, inspiring new hair growth in those sections.

In what could be the ultimate hair transplant solution, several research groups around the world are trying to make multiple copies of patients' active-stage hair follicles in the laboratory. Assuming the copies could be safely transplanted, the patient would suddenly no longer be limited to the 100,000 or so follicles he was born with; they would be supplemented by a whole crop of healthy, hair-producing look-alikes.