This domain family is found in viruses, and is typically between 156 and 171 amino acids in length. The family is found in association with pfam01443, pfam01660. 1a protein is the major virulence factor of the cucumber mosaic virus (CMV). The Ns strain of CMV causes necrotic lesions to Nicotiana spp. while other strains cause systemic mosaic. The determinant of the pathogenesis of these different strains is the specific amino acid residue at the 461 residue of the 1a protein.