ABSTRACT
Viroids are small (0.8 to 1.3 × 105 Mr), covalently circular RNAs that can be isolated from certain higher plant species afflicted with specific diseases. They replicate autonomously after introduction into healthy individuals of the same species and cause the appearance of the characteristic disease syndrome. Although many viroids were discovered because of their ability to cause readily recognizable disease symptoms in certain hosts, they may replicate in other species without causing obvious damage. In fact, it now appears that viroids are not restricted to diseased plants and may be more common in nature than previously believed. 1
