The enzyme β-galactosidase is an alternative selectable marker. When the foreign gene is inserted into the β-galactosidase gene, the enzyme β-galactosidase gets inactivated. Then the bacteria are grown on a chromogenic substrate (chromogenic substrates are peptides that react with proteolytic enzymes under the formation of colour). Non-recombinants will produce blue-coloured colonies, while recombinants will produce colourless colonies. Antibiotic resistance markers are more popular, as they tend to give lower background levels of colonies growing on selective media but not carrying the wanted gene replacement. The three most commonly used antibiotics are G418, nourseothricin (NAT), and hygromycin B (hygB).
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