The past decade has seen a dramatic increase in our understanding of the genetic basis of cancer, culminating with the award of the 1989 Nobel Prize for Medicine to Harold Varmus, MD, and J. Michael Bishop, MD, for the discovery of proto-oncogenes. Proto-oncogenes are highly conserved cellular genes, originally discovered by virtue of their homology to the cancer-causing genes (oncogenes) of tumorigenic retroviruses. While the normal function of proto-oncogenes is vital for appropriate regulation of cellular growth and differentiation, alteration of their structure or the regulation of their expression can cause cancer. In this issue of the Archives,1 Husain and colleagues, Harvard School of Dental Medicine, Cambridge, Mass, present evidence implicating overexpression of the c-Ha-ras proto-oncogene in the pathogenesis of UV light-induced photocarcinogenesis in SENCAR mice. Their results, which indicate that amplification and overexpression of the c-Ha-ras gene occurs in premalignant papillomas and malignant carcinomas induced by treatment with UV light