WD40 domain-containing proteins have 4 to 16 repeating units, all of which are thought to form a circularised beta-propeller structure (see figure to the right). The WD40 domain is composed of several repeats, a variable region of around 20 residues at the beginning followed by a more common repeated set of residues. These repeats typically form a four stranded anti-parallel beta sheet or blade. These blades come together to form a propeller with the most common being a 7 bladed beta propeller. The blades interlock so that the last beta strand of one repeat forms with the first three of the next repeat to form the 3D blade structure.
According to the initial analysis of the human genome WD40 repeats are the eighth largest family of proteins. In all 277 proteins were identified to contain them. Human genes encoding proteins containing this domain include:
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^Lander ES, Linton LM, Birren B et al. (February 2001). "Initial sequencing and analysis of the human genome". Nature409 (6822): 860–921. doi:10.1038/35057062. PMID11237011.CS1 maint: Explicit use of et al. (link)