Many proteins rely on stable, noncovalent interactions with other macromolecules to perform their function. The identification of a repeated sequence motif, the ANK repeat, in diverse proteins whose common function involves binding to other proteins indicates one way nature may achieve a wide range of protein-protein interactions. In this article, we describe evidence that these ANK repeats are involved in the specific recognition of proteins and possibly DNA, and present a model for the folding of the motif.