The origin of melanoma, one of the deadliest cancers, precedes the usual warning signs—the appearance of new or changing skin lesions. Melanoma actually begins in individual cells, most often in cells located in clear skin. But exactly which cells, under what circumstances, has been unclear. A group of scientists based at Cornell University now report that melanoma may arise from melanocyte stem cells (MCSCs), specifically, MCSCs that have become “melanoma competent” because they have accumulated a sufficient number of genetic mutations.

Merely acquiring a large enough mutational burden, however, is not enough for an MCSC to give rise to a melanoma. The MCSC must, in addition, be subjected to ultraviolet radiation strong enough to initiate a protective response—or, rather, a would-be protective response.

Ordinarily, the protective response initiated by ultraviolet (UV)-activated melanocytes involves the release of melanin, a pigment that shields the skin from the sun’s rays. …