Table 1: Some commonly used antibiotics and antibiotic resistance genes.

antibiotic	Mode of action	Resistance gene
Kanamycin	Inactivates translation by interfering with ribosome function	Neomycin or aminoglycoside phosphotransferase ( neo r) gene product inactivates kanamycin by phosphorylation
Ampicillin	Inhibits bacterial cell wall synthesis by disrupting peptidoglycan cross-linking	β-Lactamase ( amp r) gene product is secreted and hydrolyzes ampicillin
Tetracycline	Inhibits binding of aminoacyl tRNA to the 30S ribosomal subunit	tet r gene product is membrane bound and prevents tetracycline accumulation by an efflux mechanism

Blue-white screening or "lac selection" (also called a-complementation) can be used to distinguish between recombinant transformants and non- recombinant transformants. Bacterial colonies are allowed to grow on selective media containing antibiotic and X-gal(5-bromo-4-chloro-indolyl-β-D-galactopyranoside), a colorless chromogenic compound. Not all plasmid vectors are engineered for "lac selection"; the plasmid that are engineered for blue- white screening carry a MCS site in between gene that encodes for amino acids for enzyme β-galactosidase which cleaves β-glycosidic bond in D-lactose. X-gal mimic D-lactose and β-galactosidase enzyme acts on X-gal and produces a blue color complex.

A successful ligation of the desired gene disrupts the lac Z gene, hence no functional ß-galactosidase is produced resulting in white colonies. Hence successful recombinant transformed colonies can be easily identified by its white coloration from unsuccessful blue ones. pUC19, pBluescript, pGem-T are few example of cloning vectors used for this test and it also requires the use of specific E. coli host strains such as DH5α which carries the mutant lacZΔM15 genes.