The β strands are lettered sequentially with respect to the order of their occurrence in the amino acid sequence of the domains; the order in each β sheet is characteristic of immunoglobulin domains. The β strands C’ and C’’ that are found in the V domain but not in the C domains are indicated by a blue shaded background. The characteristic four-strand plus three-strand(C-region type domain) or four-strand plus five-strand(V-region type domain) arrangements are typical immunoglobulin superfamily domain building blocks, found in a whole range of other proteins as well as antibodies and T-cell receptors.