Prostate cancer is the most common male cancer in the United States, as well as in the Western World, and the second leading cause of male cancer death in the United States (1 ). The recent progress made in identifying prostate cancer genes and understanding prostate cancer genetics is impressive. However, the basic mechanisms underlying prostate carcinogenesis remain poorly understood (2 ). A hurdle in understanding the molecular genetic changes in prostate cancer has been the difficulty in establishing premalignant lesions and primary prostate tumors as in vitro cell cultures. Primary epithelial cells grow for a finite lifespan and then senesce. Immortalization is defined by continuous growth of otherwise senescing cells and is believed to represent an early stage in tumor progression. To examine these early stages, we and others have developed in vitro models of prostate epithelial cell immortalization. To understand the many factors that are suspected to contribute to the development of this malignancy, there is a need for an in vitro human prostate epithelial (HPE) culture system. These models have been extremely important in identifying genetic and molecular changes involved in prostate cancer progression.