RNA viruses are very common. An RNA virus (also known as ribovirus) uses RNA, rather than DNA, for its genetic material. RNA viruses have comparatively high mutation rates. These viruses can be either single or double strands of RNA. The RNA viruses make up four of the six classes of viruses. Many authorities consider retroviruses to be RNA viruses, but this is disputed, where other sources cite that this class of virus may also use DNA replication. Many RNA viruses are highly infectious, including pandemic transgenic species.