Human cells transformed by SV40 large T antigen achieve an extended lifespan and continue to grow and divide past the normal growth limit. This extended lifespan often ends at crisis when the cells die through fatal cell division. A few cells will survive this crisis and continue to proliferate indefinitely and are therefore considered immortal. Transformation of cells by SV40 large T antigen is associated with the induction of genomic instability at early times. This instability may contribute to a cells surviving crisis and becoming immortal through the chance disruption of genes involved in cell proliferation and regulation of cell death. Genetic instability is also observed in human tumours and the mechanisms by which it occurs both in tumour cells and SV40 transformed cells may be similar. In order to investigate these mechanisms, human and rodent cells were transfected with wild type and mutant forms of SV40 large T antigen and analyzed cytogenetically. The results of this study demonstrate that the amino terminal 147 amino acids of SV40 large T antigen are sufficient for the induction of genomic instability and at least three regions within this amino terminal fragment are necessary. One between amino acids 17 and 27. A second being the retinoblastoma protein binding site, and the third between amino acids 130 and 147. Finally, binding of T antigen to p53 appears to not be required for the induction of genomic instability, but may be necessary for the survival of aberrant cells. There is an apparent correspondence between the ability of T antigen mutants to induce genomic instability, and their abilities to induce cellular DNA synthesis and to transform and immortalize cells. / Thesis / Master of Science (MS)