The assessment of genetic changes in tissue biopsies can provide important information for prediction of tumour progression. The majority of such changes are either chromosome amplifications or deletions but there are often other complex rearrangements that can be detected using FISH. Current methodologies, namely immunohistochemistry or Southern blotting, can provide information at the gene expression level but, when carried out on tissue sections, FISH can provide information at a gene level, in situ, at the precise site within the tumour.