Numerous clinical, epidemiological and molecular findings link some types of Human Papillomaviruses (HPV) with cancer of the genital tract. They share a common pathway of transformation with a number of DNA tumour viruses, such as Adenovirus and SV40. Although all these viruses are termed 'DNA tumour viruses' and have similar in vitro transforming activities, Human Papillomavirus is the only one so far clearly involved in human cancer. Extensive studies on HPV E6 and E7 proteins have demonstrated their involvement in malignant transformation. E6 and E7 bind the products of tumour suppressor genes, p53 and Rb1, respectively, modifying or inactivating their normal functions. The Rb1 and p53 genes are deleted or mutated in several cancers and both proteins regulate the transcription of genes involved in cell cycle progression control. The E6/p53 and E7/Rb1 interactions result in a deregulation of the cell cycle with loss of control of crucial cellular events, such as DNA replication, DNA repair and apoptosis.