Functional cloning involves using a known gene sequence as a probe to look for new gene sequences that may have similar functions, based on the similarity of sequence. Using this method of analysis, researchers can search through entire genomes to find genes of interest without having prior knowledge of their location. The genes, once putatively identified, can then be tested to see if their function is in fact similar to the initial gene of interest. This technique is useful when a given pathway or enzyme is present in one organism, and one wishes to identify the presence of a similar pathway or enzyme in a different organism.
Phage expression libraries can be screened using probes designed based on the gene of interest.
Functional cloning versus positional cloning
“One way to conceptualize the difference between functional and positional cloning is to consider methods for looking up words. Functional cloning is akin to using a thesaurus to look up a known word and thereby to select new words with related meanings (or functions). Positional cloning is similar to reading though a specific page of a dictionary, browsing for interesting words of any meaning located on that particular page. In the first case, words and genes are selected for their function, be it in a sentence or in a cell. In the second case, words and genes are selected for position regardless of their function.”