for preparing cells which are suitable for transplantation therapy and which are immortal up to the time of transplantation the cells require a conditionally inducible oncogene to be present the term u201c conditionally inducible u201d is used herein to refer to oncogenes the expression of which can be regulated under certain conditions the oncogene will undergo expression when so called permissive conditions are applied for example some oncogenes are temperature sensitive and are only expressed when the temperature of their environment is below a certain value in one embodiment of the invention the oncogene that is used is a non dna binding temperature sensitive mutant of the sv 40 large t antigen gene ul9tsa58 suitable alternatives are also known and include the oncogene of the polyoma t antigen the cells also require an exogenous polynucleotide that encodes at least the catalytic sub unit of the telomerase complex the term u201c exogenous u201d is used herein in its normal context to refer to the polynucleotide introduced into the cell and distinguish from naturally occurring endogenous polynucleotides the catalytic sub unit of the telomerase complex is an enzyme that acts like a reverse transcriptase and is known in the art the human sub unit is disclosed in gb a 2317891 the oncogene and the polynucleotide encoding the telomerase may be comprised in a recombinant dna or retroviral vector or construct to transduce infect the cells the two components may be incorporated into one vector or each may be comprised in a separate.