Divergent evolution can explain how many proteins containing structurally similar domains, which perform a variety of related functions, have evolved from a relatively small number of modules or protein domains. However, it cannot explain how protein domains with similar, but distinguishable, functions and similar, but distinguishable, structures have evolved. Examples of this are the RNA-binding protein containing the RNA-binding domain (RBD), and a newly established protein group, the cold-shock domain (CSD) protein family. Both protein domains contain conserved RNP motifs on similar single-stranded nucleic acid-binding surfaces. Apart from the RNP motifs, which have a similar function, the two families show little similarity in topology or amino acid sequence. This can be considered an interesting example of convergent evolution at the molecular level. Previously, a beta-sheet surface was found to interact with RNA in non-homologous proteins from yeast, phage and man, revealing that this mode of RNA binding may be a widely recurring theme.