We have identified a family of 'Agenet' domains that are plant-specific homologs of Tudor domains. This finding has been extended, using a combination of sequence- and structure-dependent approaches, to show that the three beta-stranded core regions of Tudor, PWWP, chromatin-binding (Chromo) and MBT domains are homologous because they originate from a common ancestor. In addition, we have revealed pairs of tandem repeats in the fragile X mental retardation protein (FMRP) family that are also members of this Tudor domain 'Royal Family'.