Recombinant DNA technology involves several steps in specific sequence such as isolation of DNA, fragmentation of DNA by restriction endonucleases, isolation of a desired DNA fragment, ligation of the DNA fragment into a vector, transferring the recombinant DNA into the host, culturing the host cells in a medium at large scale and extraction of the desired product. The blue–white screen is a screening technique that allows for the rapid and convenient detection of recombinant bacteria in such experiments. Recombinant DNA is inserted into a competent host cell viable for transformation, which are then grown in presence of X – gal. Cell transformed with vectors containing Recombinant DNA will produce white colonies, cells transformed with non – recombinant plasmids (i.e. only the vector) grow into blue colonies. This method of screening is usually performed using a suitable bacterial strain, but other organisms such as yeast may also be used.