A pseudo-dyad was found to exist in the monomers of the crystal structures of the proteinases from Rous sarcoma virus and the human immunodeficiency virus. This dyad, also discovered earlier in pepsin-like aspartic proteinases and considered to be of probable evolutionary origin, has been shown to arise as a result of the topology and the folding of the proteinase monomers and may not therefore have much evolutionary significance.
