Covariation analyses of U/C motif in viroids of genus Pospivitoid, based on conservation of the overall secondary structures predicted by mfold. The analyses reveal conservation of U/C pair at the equivalent positions for Tomato planta macho viroid (TPMVd), Tomato chlorotic dwarf viroid (TCDVd), variation C/C for Citrus Exocortic viroid (CEVd), Tomato apical stunt viroid (TASVd) and Chrysanthemum stunt viroid (CSVd), and variation A/A for Mexican papita viroid (MPVd).