LB medium is a rich medium that is commonly used to culture members of the Enterobacteriaceae as well as for coliphage plaque assays. LB and related media (SOC, Terrific Broth, 2xYT, etc) are used extensively in recombinant DNA work and other molecular biology procedures. Often an antibiotic is added to the sterilized medium to select for cells that contain a specific genetic element such as a plasmid, a transposon, or a gene disruption via an antibiotic resistance cassette. X-Gal (5-bromo-4-chloro-3-indolyl-beta-D-galactoside) may be added to sterile medium when using the blue-white screen for plasmids bearing the alpha fragment of the beta-galactosidase gene (a-complementation analysis). IPTG (isopropyl-beta-D-thiogalactopyranoside) is sometimes added to induce expression of genes controlled by the lacpromoter. In addition to (or perhaps because of) its prominent position in the molecular biology field, LB has also been used as a general-purpose bacterial culture medium for a variety of facultative organisms. In the undergraduate microbiology teaching labs, LB is sometimes used as the growth surface when attempting to analyze bacterial colony morphology.