The ankyrin repeat is one of the most common protein sequence motifs. Recen
t X-ray and MMR structures of ankyrin-repeat proteins and their complexes h
ave provided invaluable insights into the molecular basis of the extraordin
ary variety of biological activities of these molecules. In particular, the
y have begun to reveal how a large family of structurally related proteins
can interact specifically with such a diverse array of macromolecular targe
ts.