<p>The basic structure of immunoglobulin (Ig) molecules is a tetramer of two light chains and two heavy chains linked by disulphide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains: alpha, delta, epsilon, gamma and mu, all consisting of a variable domain (VH) and three (in alpha, delta and gamma) or four (in epsilon and mu) constant domains (CH1 to CH4). Ig molecules are highly modular proteins, in which the variable and constant domains have clear, conserved sequence patterns. The domains in Ig and Ig-like molecules are grouped into four types: V-set (variable; <db_xref db="INTERPRO" dbkey="IPR013106"/>), C1-set (constant-1; <db_xref db="INTERPRO" dbkey="IPR003597"/>), C2-set (constant-2; <db_xref db="INTERPRO" dbkey="IPR008424"/>) and I-set (intermediate; <db_xref db="INTERPRO" dbkey="IPR013098"/>) [<cite idref="PUB00014840"/>]. Structural studies have shown that these domains share a common core Greek-key beta-sandwich structure, with the types differing in the number of strands in the beta-sheets as well as in their sequence patterns [<cite idref="PUB00015110"/>, <cite idref="PUB00010610"/>].</p><p>Immunoglobulin-like domains that are related in both sequence and structure can be found in several diverse protein families. Ig-like domains are involved in a variety of functions, including cell-cell recognition, cell-surface receptors, muscle structure and the immune system [<cite idref="PUB00027656"/>]. </p><p>This entry represents C1-set domains, which are classical Ig-like domains resembling the antibody constant domain. C1-set domains are found almost exclusively in molecules involved in the immune system, such as in immunoglobulin light and heavy chains, in the major histocompatibility complex (MHC) class I and II complex molecules [<cite idref="PUB00015111"/>, <cite idref="PUB00015113"/>], and in various T-cell receptors.</p>