Our knowledge of the large family of proteins that contain the WD repeat continues to accumulate. The WD-repeat proteins are found in all eukaryotes and are implicated in a wide variety of crucial functions. The solution of the three-dimensional structure of one WD-repeat protein and the assumption that the structure will be common to all members of this family has allowed subfamilies of WD-repeat proteins to be defined on the basis of probable surface similarity. Proteins that have very similar surfaces are likely to have common binding partners and similar functions.