This is a large family of putative suppressors of RNA silencing proteins, P20-P25, from ssRNA positive-strand viruses such as Closterovirus, Potyvirus and Cucumovirus families. RNA silencing is one of the major mechanisms of defense against viruses, and, in response, some viruses have evolved or acquired functions for suppression of RNA silencing. These counter-defencive viral proteins with RNA silencing suppressor (RSS) activity were originally discovered in the members of plant virus genera Potyvirus and Cucumovirus. Each of the conserved blocks of amino acids found in P21-like proteins corresponds to a computer-predicted alpha-helix, with the most C-terminal element being 42 residues long. This suggests conservation of the predominantly alpha-helical secondary structure in the P21-like proteins.