<p>This entry represents domains with an immunoglobulin-like (Ig-like) fold, which consists of a beta-sandwich of seven or more strands in two sheets with a Greek-key topology. Ig-like domains are one of the most common protein modules found in animals, occurring in a variety of different proteins. These domains are often involved in interactions, commonly with other Ig-like domains via their beta-sheets [<cite idref="PUB00003330"/>, <cite idref="PUB00018310"/>, <cite idref="PUB00015110"/>, <cite idref="PUB00027656"/>]. Domains within this fold-family share the same structure, but can diverge with respect to their sequence. Based on sequence, Ig-like domains can be classified as V-set domains (antibody variable domain-like), C1-set domains (antibody constant domain-like), C2-set domains, and I-set domains (antibody intermediate domain-like). Proteins can contain more than one of these types of Ig-like domains. For example, in the human T-cell receptor antigen CD2, domain 1 (D1) is a V-set domain, while domain 2 (D2) is a C2-set domain, both domains having the same Ig-like fold [<cite idref="PUB00027700"/>].</p><p>Domains with an Ig-like fold can be found in many, diverse proteins in addition to immunoglobulin molecules. For example, Ig-like domains occur in several different types of receptors (such as various T-cell antigen receptors), several cell adhesion molecules, MHC class I and II antigens, as well as the hemolymph protein hemolin, and the muscle proteins titin, telokin and twitchin. </p>