A technique called in situ hybridization shows whether a gene is expressed in cells and also provides clues about the function of the gene. This technique has helped identify activated oncogenes in cancer cells and their normal counterparts in normal cells, in many different species. In this photograph, a labeled segment of DNA (a known oncogene) has been placed into a mouse oocyte, a cell that develops into a mature egg. The labeled DNA paired with (or hybridized to) multiple copies of RNA in the mouse oocyte. The presence of this RNA (shown here as black dots inside the nucleus of the immature cell) shows that the normal cellular counterpart of the oncogene is active, suggesting that it is essential for normal cell development germline. Gene expression is manifested by the production of RNA transcripts in cells. Hybridization histochemistry (in situ hybridization) allows localization of these transcripts with cellular or higher resolution. Furthermore, the relative amounts of transcr