Human retroviruses are associated with a wide spectrum of clinical entities including cancers, immune deficiency and neurological disorders. They have become the focal point of all retrovirology by virtue of their extreme clinical relevance, their novel and complex biologic and genetic properties, as well as their regulation strategies. The study of these viruses is of great importance as understanding of their interactions with the host will ultimately shed light on fundamental mechanisms of genetic controls in human cells in their normal state and the alterations in these controls in neoplastic or immunologically aberrant states.