TOP1 and neoplasm: Measuring gene copy number alterations by FISH traditionally relies on the use of a same chromosome reference probe, e.g. using CEN-20 for measuring genes on chromosome 20, we therefore set out to develop a novel FISH assay to distinguish tumor specimens with TOP1 copy number increases due to amplifications from those with increases due to 20q gain or aneusomy by applying a reference probe directed at an unrelated chromosome.