Members of the IgSF have diverged in sequence and function. However, all contain domains with the characteristic immunoglobulin fold, which is defined by two opposing antiparallel b-sheets connected in a unique manner [1, 2] . The core of the immunoglobulin fold is formed by four b-strands (B, C, E, and F) augmented with three to five additional b-strands (A, C9, C0, D, and G) to yield several distinct subtypes [1, 2] . Most common are the Vset and C-set immunoglobulin domains, which are named according to their occurrence in the variable and constant regions of immunoglobulins, respectively. A third type, the I-set, is an intermediate structure between the V-and C-sets found frequently in cell-surface receptors. Immunoglobulin domains rarely occur in isolation but typically form concatenated chains, often with a V-set or I-set domain at the N-terminus.