The development of dominant selection markers to identify eukaryotic cells that have undergone a gene transformation event has greatly facilitated molecular genetic studies in higher eukaryotic cells. Selection schemes based on resistance to antibiotic cytotoxicity (1 ,2 ) will be described in this chapter. Other schemes—for example, based on resistance to inhibition of DNA synthesis by methotrexate (1 ) or mycophenolic acid (1 )—are described in other chapters of this book. Prior to the development of dominant selection markers, the use of recessive markers, such as thymidine kinase (TK) or hypoxanthine-guanine phosphoribosyl transferase (HGPT) was limited to a handful of mutant cell lines that were TK − or HGPT− (5 ,6 ). If one wished to transfect a wild-type cell line, one had first to select a recessive mutant derivative cell line and characterize it before proceeding with the experiments of interest. Such restrictions posed a significant barrier to molecular genetic analyses in higher eukaryotic cells.