Going bald is the one big vanity concern among most men. But relax, it doesn't need to be that way; scientists have just worked out exactly why men go bald, and already have a way to stop it happening.
The discovery could mean treatments are developed to suppress the protein and to stop baldness, although it would not reverse the effects to reverse hair loss.
Tests were carried out on tissue from the scalps of more than 20 men with male pattern baldness, known as androgenic alopecia (AGA).
The results showed bald areas had levels of the protein PGD2 three times higher than hairy areas.
There are already 10 drugs available that can block the receptor that allowed PGD2 to work, which could help researchers develop a treatment in the form of a cream or ointment.
The research, led by Professor George Cotsarelis from the University of Pennsylvania, was published in the journal Science Translational Medicine.