Fluorescence mapping of an oncogene. Fluorescence light micrograph showing a close-up of cell nuclei, mapping the position of genes. This research aids understanding of what governs the genome. Here, a single gene called PEM (polymorphic epithelial mucin, purple) has been localized using fluorescence in-situ hybridization. DNA is stained blue, while the cell cytoplasm is stained green. Overexpression of PEM (also called MUC1) is often associated with colon, breast, ovarian, lung and pancreatic cancers. An oncogene is a gene that causes cancer.