As detection of relationships improves with more samples and better methodology, families and super- families can become more populated. At present, structur- al relationships provide the highest level of classification, and structure-based databases classify proteins with similar ‘folds’. These classifications reveal that whenever structures are known for two proteins that are considered members of the same family or superfamily, the structures are similar, whereas the converse is often not true. Therefore, significant sequence similarity can be used to infer common structure (and common ancestry); however, similar structures that lack detectable sequence similarity may have resulted fromeither divergence beyond detection or convergence to a similar divergence froma common ancestor can occur with retention of.
