Recent developments toward a unifying concept of carcinogenesis

Although considerable progress has been made toward identification of the various factors involved in the causation of cancer, little is known about how they mediate the neoplastic transformation. Recent developments in the molecular biology of tumor viruses and cancer genetics have established that normal mammalian cells contain genes sequences (oncogenes) that are homologous to those in certain oncogenic RNA viruses in mouse, rat, and chicken. These sequences are turned off or expressed at very low levels of activity in normal cells and conversely turned on in the neoplastic state where, in some instances, large amounts of a specific gene product are produced which appears to be linked to the neoplastic transformation. Oncogenes may represent the final common pathway through which chemicals, ionizing radiation, oncogenic viruses ultimately cause cancer.