We report the discovery of a novel family of proteins, each member contains tandem pentapeptide (five residue) repeats, described by the motif A(D/N)LXX. Members of this family are both membrane bound and cytoplasmic. The function of these repeats is uncertain, but they may have a targeting or structural function rather than enzymatic activity. This family is most common in cyanobacteria, suggesting a function related to cyanobacterial-specific metabolism. Although no experimental information is available for the structure of this family, it is predicted that the tandem pentapeptide repeats will form a right-handed beta-helical structure. A structural model of the pentapeptide repeats is presented.