The principles of recombinant DNA technology are illustrated based on the biology of somatic cells with transcription of chromosomal DNA into messenger RNA and its translation into protein. Recombinant DNA technology allows the cloning, identification and characterization of genes and their expression with the production of medically relevant proteins. The products of recombinant DNA technology are cloned genes on the one hand. These may be useful for molecular diagnosis and gene therapy. On the other hand they comprise recombinant proteins which may be useful in the diagnosis, therapy and prevention of human diseases.