Any of the covalently closed dna molecules found in bacteria, many viruses, mitochondria, plastids, and plasmids. Small, polydisperse circular dna's have also been observed in a number of eukaryotic organisms and are suggested to have homology with chromosomal dna and the capacity to be inserted into, and excised from, chromosomal dna. It is a fragment of dna formed by a process of looping out and deletion, containing a constant region of the mu heavy chain and the 3'-part of the mu switch region. Circular dna is a normal product of rearrangement among gene segments encoding the variable regions of immunoglobulin light and heavy chains, as well as the T-cell receptor. (Riger et al., Glossary of genetics, 5th ed & Segen, dictionary of Modern medicine, 1992)