Amongst the most intruiging protein folds, the propeller fold comes in a number of varieties. Shown here is a 7 bladed version, but they occur in 3 to 8 bladed varieties. Note that this fold is constructed from a single chain, rather than from 7 identical units, although this fold almost certainly arose from internal duplications of gene segments. Many hundreds of protein structures with this fold have been determined. In the case of enzymes the enzyme's active site is often found in the cleft formed in the center of the propeller by loops connecting the successive four-sheet motifs. Examples include the the influenza virus protein neuraminidase - one of two proteins present in the viral envelope. It catalyzes the cleavage of sialic acid moieties from cell-membrane proteins to aid in the targeting of newly produced virions to previously uninfected cells.