Hepatitis B virus
Depending on whether a lax or strict definition is used, the term "virusoid" may also include Hepatitis D virus (HDV). Like plant virusoids, HDV is circular, single-stranded, and supported by a helper virus (Hepatitis B virus) to form virions; however, the virions possess a much larger genome size (~1700 nt) and encode a protein. They also show no sequence similarity with the plant virusoid group.