According to the central dogma, DNA codes for mRNA, which codes for protein. An exception to this rule is seen with the retroviruses, RNA-encoded viruses that have a phase in their replication cycle during which their genomic RNA is copied into DNA by a virally-encoded enzyme known as reverse transcriptase. The ability to convert RNA to DNA can be useful in the laboratory. For example, the power of PCR can be brought to RNA by converting RNAs of interest to DNA and then amplifying them by PCR. With reverse transcriptase, this is readily accomplished. First, one creates a DNA oligonucleotide to serve as a primer for reverse transcriptase to use on a target RNA. The primer must, of course, be complementary to a segment (near the 3’ end) of the RNA to be amplified. The RNA template, reverse transcriptase, the primer, and four dNTPs are mixed. With one round of replication, the RNA is converted to a single strand of DNA, which can be separated from the RNA either by heating or by the use of an RNase to digest the RNA. The product of this process is called a complementary DNA (cDNA).