Gel electrophoresis is a molecular technique used for the separation and analysis of DNA fragments. The separated DNA fragments can be visualised only after staining the DNA with a compound known as ethidium bromide followed by exposure to UV radiation. Bright orange coloured bands of DNA can be observed in a ethidium bromide stained gel when exposed to UV light. The separated bands of DNA are cut out from the agarose gel and extracted from the gel piece. This step is known as elution. Selectable markers can differentiate recombinants from non-recombinants due to their capacity to form colour in presence of a chromogenic substance. The presence of a chromogenic substrate gives blue colored colonies if the plasmid in the bacteria does not have an insert.