to RCR. Parvovirus genomes have hairpin loops at each end of the genome that repeatedly unfold and refold during replication to change the direction of DNA synthesis to move back and forth along the genome, producing numerous copies of the genome in a continuous process. Individual genomes are then excised from this molecule by the viral endonuclease. For parvoviruses, either the positive or negative sense strand may be packaged into capsids, varying from virus to virus. Nearly all ssDNA viruses have positive sense genomes, but a few exceptions and peculiarities exist. The family Anelloviridae is the only ssDNA family whose members have negative sense genomes, which are circular. Parvoviruses, as previously mentioned, may package either the positive or negative sense strand into virions. Lastly, bidnaviruses package both the positive and negative linear strands. In any case, the sense of ssDNA viruses, unlike for ssRNA viruses, is not sufficient to separate ssDNA viruses into two groups since all ssDNA viral genomes are converted to dsDNA forms prior to transcription and replication. ssDNA viruses are classified into one of the four realms and include several families that are unassigned to a realm: In Monodnaviria, all members except viruses in Papovaviricetes are ssDNA viruses. The unassigned families Anelloviridae and Spiraviridae are ssDNA virus families. Viruses in the family Finnlakeviridae contain ssDNA genomes. Finnlakeviridae is unassigned to a realm but is a proposed member of Varidnaviria. RNA viruses RNA viruses have genomes made of ribonucleic acid (RNA) and comprise three groups: double-stranded RNA (dsRNA) viruses, positive sense single-stranded RNA (+ssRNA) viruses, and negative sense single-stranded RNA (-ssRNA) viruses. The majority of RNA viruses are classified in the kingdom Orthornavirae in the realm Riboviria. The exceptions are generally viroids and other subviral agents. Some of the latter category, such as the hepatitis D virus, are classified in the realm Ribozyviria. Group III: double-stranded RNA viruses The third Baltimore group contains viruses that have a double-stranded RNA (dsRNA) genome. After entering a host cell, the dsRNA genome is transcribed to mRNA from the negative strand by the viral RNA-dependent RNA polymerase (RdRp). The mRNA may be used for translation or replication. Single-stranded mRNA is replicated to form the dsRNA genome. The 5′-end of the genome may be naked, capped, or covalently bound to a viral protein. dsRNA is not a molecule made by cells, so cellular life has evolved antiviral systems to detect and inactivate viral