A technique used for searching for a specific dNA fragment. The process is as follows:
1. separate DNA fragments by gel electrophoresis
2. Change pH of gel to basic, thus allowing disruption of H-bonds
3. blot gel with nitrocellulose paper
4. Heat paper so as to fix dNA fragments
5. probe with labelled messenger RNA or cDNA
complementary mRNA/cDNA fragments will have hybridised.