<unk>  (covalently closed circular DNA) is a special DNA structure that arises during the propagation of some viruses in the cell nucleus and may remain permanently there. It is a double-stranded DNA that originates in a linear form that is ligated by means of DNA ligase to a covalently closed ring. In most cases, transcription of viral DNA can occur from the circular form only. The<unk>  of viruses is also known as episomal DNA or occasionally as a minichromosome.<unk>  was first described in bacteriophages, but it was also found in some cell cultures where an infection of DNA viruses (Polyomaviridae) was detected.<unk>  is typical of Caulimoviridae and Hepadnaviridae, including the hepatitis B virus (HBV).<unk>  in HBV is formed by conversion of capsid-associated relaxed circular DNA (rcDNA). Following hepatitis B infections,<unk>  can remain following clinical treatment in liver cells and can rarely reactivate. The relative quantity of<unk>  present is an indicator for HBV treatment.
CccDNA
TRNA 
SV40 large T antigen
Mutanome
Replicon 
Gamma probe
Long interspersed nuclear element
Triage tag