marker such that those cells without the plasmid may be killed or have their growth arrested. antibiotic resistance is the most commonly used marker for prokaryotes. the transforming plasmid contains a gene that confers resistance to an antibiotic that the bacteria are otherwise sensitive to. the mixture of treated cells is cultured on media that contain the antibiotic so that only transformed cells are able to grow. another method of selection is the use of certain auxotrophic markers that can compensate for an inability to metabolise certain amino acids, nucleotides, or sugars. this method requires the use of suitably mutated strains that are deficient in the synthesis or utility of a particular biomolecule, and the transformed cells are cultured in a medium that allows only cells containing the plasmid to grow. in a cloning experiment, a gene may be inserted into a plasmid used for transformation. however, in such experiment, not all the plasmids may contain a successfully inserted gene. additional techniques may therefore be employed further to screen for transformed cells that contain plasmid with the insert. reporter genes can be used as markers, such as the lacz gene which codes for β-galactosidase used in blue-white screening. this method of screening relies on the principle of α-complementation, where a fragment of the lacz gene (laczα) in the plasmid can complement another mutant lacz gene (laczδm15) in the cell. both genes by themselves produce non-functional peptides, however, when expressed together, as when a plasmid containing lacz-α is transformed into a laczδm15 cells, they form a functional β-galactosidase. the presence of an active β-galactosidase may be detected when cells are grown in plates containing x-gal, forming characteristic blue colonies. however, the multiple cloning site, where a gene of interest may be ligated into the plasmid vector, is located within the laczα gene. successful ligation therefore disrupts the laczα gene, and no functional β-galactosidase can form, resulting in white colonies. cells containing successfully ligated insert can then be easily identified by its white coloration from the unsuccessful blue ones. other commonly used reporter genes are green fluorescent protein (gfp), which produces cells that glow green under blue light, and the enzyme luciferase, which catalyzes a reaction with luciferin to emit light. the recombinant dna may also be detected using other methods such as nucleic acid hybridization with radioactive rna probe, while cells that expressed the desired protein from the plasmid may also be detected using immunological methods.