Approximately 30 species of viroids are currently known and classified into two families: Pospiviroidae and Avsunviroidae. The latter are characterized by the presence in their sequence of a hammerhead ribozyme (HHR) which is a small catalytic RNA motif involved in their replication through a symmetric rolling-circle mechanism (for recent reviews see Ding
[3],
Owens & Hammond [4] and Flores et al. [5]). Actually, the presence of hammerhead ribozymes is not restricted to the viroids and they are largely distributed in the genomes of all kinds of organisms including human