IGSF21 is a 467 amino acid secreted protein that contains two Ig-like (immunoglobulin-like) domains and is a member of the immunoglobulin superfamily. Members of this family of proteins usually localize to the cell membrane, and may act as receptors in the immune response pathways. The gene encoding IGSF21 maps to human chromosome 1p36.13 and mouse chromosome 4 D3. Human chromosome 1 spans 260 million base pairs, contains over 3,000 genes and comprises nearly 8% of the human genome. A large number of disease-associated genes are associated with chromosome 1, including those that are involved in familial adenomatous polyposis, Stickler syndrome, Parkinson’s disease, Gaucher disease, schizophrenia and Usher syndrome. Aberrations in chromosome 1 are found in a variety of cancers, including head and neck cancer, malignant melanoma and multiple myeloma.