Ankyrin repeat proteins are a class of naturally occurring, non-antibody binding proteins characterized by a highly conserved structural motif known as the ankyrin repeat, which consists of approximately 33 amino acids folded into a helix-turn-helix conformation. These modular repeats typically stack together in tandem arrays to form larger structural domains that mediate critical protein-protein interactions in nature across diverse organisms, including humans. Because of their rigid framework and naturally hypervariable surface residues involved in binding other molecules, scientists use them as natural scaffolds to engineer custom binding proteins tailored for specific medical and diagnostic applications.