Cancer cell nucleolus. Coloured transmission electron micrograph (TEM) of an abnormally large nucleolus (black) in the nucleus (green) of a testicular cancer cell. The nucleolus is the most active part of the nucleus and is the site of ribosomal RNA and ribosome protein synthesis. Because cancer cells are unusually active they often show abnormally large or irregular nuclei and nucleoli. Magnification: x4000 when printed 10 centimetres wide.