Mammalian cells normally have a limited life span and after having undergone approximately 30 population doublings stop dividing; this is known as senescence. However, when the control mechanisms which limit life span are modified cells can become immortalized i.e. have an unlimited growth potential (1). One such way of subverting these control mechanisms is to express an immortalizing oncogene in normal cells. These proteins override the normal control points and one of the most studied and best understood of this class of oncogenes is large T antigen which is contained within the genome of the monkey virus SV40 (2). SV40 large T antigen binds a number of proteins, but the most important are p53 and Rb which, when functioning normally, are negative regulators of cell division. Upon being bound by SV40 large T antigen, p53 and Rb are inactivated and the cells have an unlimited life span. There are a number of good reviews on immortalization (3–5) that deal in greater detail with biology involved in the process which is beyond the scope of this review.