WD proteins are made up of highly conserved repeating units usually ending with Trp-Asp (WD). They are found in all eukaryotes but not in prokaryotes. They regulate cellular functions, such as cell division, cell-fate determination, gene transcription, transmembrane signalling, mRNA modification and vesicle fusion. Here we define the common features of the repeating units, and criteria for grouping such proteins into functional subfamilies.