Viroids are small circular RNAs (246-401 nt) that infect plants.Despite their small size and lack of protein-coding any ability, they are able to complete a complex infectious cycle in the host plant.The more than 30 viroid species currently known are classified in two families: Pospiviroidae, including viroids that replicate in the nucleus through an asymmetric rolling-circle mechanism, and Avsunviroidae, including viroids that replicate in the chloroplast through a symmetric rolling-circle mechanism.