It is generally agreed that sunlight exposure is one of the etiologic agents in
malignant melanoma of fair-skinned individuals. However, the wavelengths responsible for
tumorigenesis are not known, although
DNA is assumed to be the target because individuals defective in the repair of UV damage to
DNA are several thousandfold more prone to the disease than the average population. Heavily pigmented backcross hybrids of the genus Xiphophorus (platyfish and swordtails) are very sensitive to
melanoma induction by single exposures to UV. We irradiated groups of five 6-day-old fish with narrow wavelength bands at 302, 313, 365, 405, and 436 nm and scored the irradiated animals for
melanomas 4 months later. We used several exposures at each wavelength to obtain estimates of the sensitivity for
melanoma induction as a function of exposure and wavelength. The action spectrum (sensitivity per incident photon as a function of wavelength) for
melanoma induction shows appreciable sensitivity at 365, 405, and probably 436 nm, suggesting that wavelengths not absorbed directly in
DNA are effective in induction. We interpret the results as indicating that light energy absorbed in
melanin is effective in inducing
melanomas in this animal model and that, in natural sunlight, 90-95% of
melanoma induction may be attributed to wavelengths > 320 nm--the UV-A and visible spectral regions.