(A) A classical cadherin molecule. The protein is a homodimer, with the extracellular part of each polypeptide folded into five cadherin repeats. There are Ca²⁺-binding sites between each pair of repeats. (B) The crystal structure of a single cadherin repeat, which resembles an immunoglobulin (Ig) domain. (C) The influence of extracellular Ca²⁺. As the amount of Ca²⁺ increases, the extracellular parts of the cadherin chains become more rigid. When enough Ca²⁺ is bound, the cadherin dimer extends from the surface, where it can bind to a cadherin dimer on a neighboring cell. If Ca²⁺ is removed, the extracellular part of the protein becomes floppy and is degraded by proteolytic enzymes.