<p>WD-40 repeats (also known as WD or beta-transducin repeats) are short ~40 amino acid motifs, often terminating in a Trp-Asp (W-D) dipeptide. WD40 repeats usually assume a 7-8 bladed beta-propeller fold, but proteins have been found with 4 to 16 repeated units, which also form a circularised beta-propeller structure. WD-repeat proteins are a large family found in all eukaryotes and are implicated in a variety of functions ranging from signal transduction and transcription regulation to cell cycle control and apoptosis. Repeated WD40 motifs act as a site for protein-protein interaction, and proteins containing WD40 repeats are known to serve as platforms for the assembly of protein complexes or mediators of transient interplay among other proteins. The specificity of the proteins is determined by the sequences outside the repeats themselves. Examples of such complexes are G proteins (beta subunit is a beta-propeller), TAFII transcription factor, and E3 ubiquitin ligase [<cite idref="PUB00015237"/>, <cite idref="PUB00005491"/>]. In Arabidopsis spp., several WD40-containing proteins act as key regulators of plant-specific developmental events.</p><p>The structures of several WD40 repeat-containing proteins have been determined, including the beta-1 subunit of the signal-transducing G protein heterotrimer, the C-terminal domain of yeast Tup1, the C-terminal domain of Groucho/tle1, the Cdc4 propeller domain, the bovine Arp2/3 complex 41 kDa subunit ARPC1, and actin interacting protein 1. </p>