Examples of tumours with increased gene copy number detected by FISH. Gene copy number visualized by fluorescent in situ hybridization (FISH). Blue DAPI stain identifies the DNA present in each cell's nucleus. Red Cy5-labelled probes hybridize to the gene region targeted by each assay (EGFR or HER2). Green Cy3-labelled probes target the centromere of the chromosome appropriate for the gene-specific assay (chromosome 7 for EGFR, chromosome 17 for HER2). The ratio reported is the number of red probes/green probes (genes/chromosome) based on an average of 40 cells. A) Tumour cell without increased EGFR copy number, B) Tumour cell with increased HER2 copy number C) Tumour cell with greatly amplified EGFR.