A technique developed by $EM$ Southern in $1975$ for detection of a specific $DNA$ sequences (gene or other) in a large, complex sample of $DNA$ $(e . g .$, cellular $DNA$). It is also used to determine the molecular weight of a restriction fragment and to measure relative amounts in different sample. Uses Southern blots are used in gene discovery and mapping, evolution and development studies, diagnostics and forensics.In regards to genetically modified organisms, Southern blotting is used as a definitive test to ensure that a particular section of $DNA$ of known genetic sequence has been successfully incorporated into the genome of the host organism