Abstract: The present invention relates to a method of diagnosis or prognosis of tumor cells in a patient displaying a mutator phenotype in the tumor which comprises the steps of: a) obtaining a genomic DNA sample of the tumor; b) obtaining a genomic DNA sample of a tumor-free tissue of the patient; subjecting the DNA samples of steps a) and b) to amplification using primers which are flanking repeat pattern affected by a mutator phenotype or cold inter-Alu PCR followed by hybridization with a probe corresponding to an instability prone locus; d) subjecting the amplified fragments of step c) to electrophoretic fractionation on a polyacrylamide gel to determine the presence of a variation in band profile between tumor and tumor-free DNA indicating a genomic instability associated with a mutator phenotype; and e) employing means for comparing the allelic status of at least one instability prone locus in the DNA samples of step c).