Evidence that pyrimidine dimers in DNA can give rise to tumors.

The Amazon molly, Poecilia formosa, is a small fish that grows in clones. Hence, cells from one animal may be transplanted to another without danger of rejection. Cells from thyroid and adjacent tissue were irradiated with UV light in vitro and injected into the abdominal cavity of isogeneic recipients. At appropriate UV doses and numbers of cells injected, all recipients showed exuberant thyroid proliferation. We give arguments and data indicating that the proliferation is a tumor, not a goitrogenic response. If the UV irradiation is followed, but not preceded, by photoreactivating illumination, the yield of thyroid growths is markedly decreased. Because other investigations have shown that photoreactivation monomerizes UV-induced cyclobutylpyrimidine dimers in DNA and does not affect other photoproducts, our data indicate that pyrimidine dimers in DNA can give rise to tumors.