C17orf99 (Chromosome 17 Open Reading Frame 99) is located on human chromosome 17q25.3. The C17orf99 protein is biasedly expressed in bone marrow and lymph node. Structurally, the encoded protein is reported to be 27160 Da in mass. The C17orf99 (also known as IL-40; UNQ464) gene is conserved in chimpanzee, dog, mouse, and rat. 91 organisms have orthologs with human gene C17orf99. C17orf99 is a probable B cell-associated cytokine that plays a role in the regulation of humoral immune responses. C17orf99 is involved in lymphocyte B cell development and immunoglobulin/IgA production. C17orf99 is associated with some diseases, such as Amelogenesis Imperfecta, Type Ie. An important paralog of this gene is FCRL5. C17orf99 encodes a novel B cell-associated cytokine that plays an important role in humoral immune responses and may also play a role in B cell development. Importantly, IL-40 is also expressed by human activated B cells and by several human B cell lymphomas. IL-40 may play a role in the pathogenesis of certain human diseases.