The transformation of eukaryotic cells by DNA is often called transfection. This can be demonstrated using a genetic marker, a gene whose presence in the recipient cells confers an observable phenotype. When transforming both prokaryotes and eukaryotes, researchers often use a genetically determined selection marker, for example, antibiotic resistance or a nutritional requirement, which permits the growth of transformed cells but not of nontransformed cells. A common marker in mammalian transfection experiments is a gene that confers resistance to the antibiotic neomycin. Transfection is achieved by various methods, including chemical treatments that allow the DNA to be taken up by the cells. Any cell can be transfected, even an egg cell. In the latter case, a whole new genetically transformed organism can result, known as a transgenic organism. Transformation in eukaryotes offers conclusive evidence that DNA is the genetic material.