Members of a family of proteins containing a conserved approximately 80-amino acid RNA recognition motif (RRM) bind specifically to a wide variety of RNA molecules. Structural studies, in combination with sequence alignments, indicate the structural context of both conserved and non-conserved elements in the motif. These analyses suggest that all RRM proteins share a common fold and a similar protein-RNA interface, and that non-conserved residues contribute additional contacts for sequence-specific RNA recognition.