Amino acid residues, labeled in red represenr amino acid replacements found in mutants. Using serological techniques these replacements are detectable in immunological assays. It was argued that replacements in these positions change surface properties of the virus particle (change of antigenic properties). Positions labeled in blue do not change the surface and thus remain immunologically undetectable (hidden antigens). The RNA-section in contact with one protein subunit is shown in orange (P. v. SENGBUSCH, 1965, 1983)