Light micrograph of a mouse oocyte that is expressing an oncogene (a cancer-causing gene). The technique being used to test its expression is called in situ hybridization. This involves creating and labelling in the laboratory a single-stranded DNA section that includes the oncogene, and introducing it to the cell. Here it is bonding to the single-stranded RNA (seen as black dots) made by the natural oncogene inside the cell nucleus. This shows that this oncogene is active and suggests it is critical for normal germ cell development.