As the protein sequence and structure databases expand rapidly a better understanding of the relationships between proteins is required. A classification is considered that extends the sequence-based superfamilies to include proteins with similar function and three-dimensional structures but no sequence similarity. So far there are only nine protein folds known to recur in proteins having neither sequence nor functional similarity. These folds dominate the structure database, representing more than 30 per cent of all determined structures. This observation has implications for protein-fold recognition.