Restriction enzymes are proteins that are produced or manufactured by bacteria for use as defense systems against viruses. Anything that is restrictive, or termed restrictive, means it will allow access only to qualified organisms, not imitations, and certainly not foreign invaders, such as viruses. Defense mechanisms of this sort employ a modification of the bacterial DNA by methylation, which is the addition of a methyl group (CH3) to a specific location on the DNA. The biotechnical applications that make use of this techniique are cloning and genetic mapping. The addition of the methyl groups in the process of methylation splits the bacterial DNA into shorter, specific sections of DNA that may be easily marked in the whole genome by genetic mapping. This makes it easier to select and isolate specific sections of DNA from the genome as a whole.