Family with sequence similarity 3 (FAM3) family is a novel cytokine-like gene family, which has four genes in this family, FAM3A, FAM3B, FAM3C, and FAM3D, each encoding a protein (224-235 amino acids) with a hydrophobic leader sequence. It had indicated that FAM3B/PANDER (pancreatic derived factor) is highly expressed in pancreas, and FAM3A and FAM3C in almost all tissues. FAM3D is abundantly expressed in placenta and weakly expressed in small intestine. Immunohistochemistry showed that FAM3A is expressed prominently in the vascular endothelium, particularly capillaries. FAM3A and FAM3B protein were both localized to the islets of Langerhans of the endocrine pancreas. Recombinant FAM3B protein has delayed effects on beta-cell function. FAM3C is involved in retinal laminar formation processes in vertebrates. NFATC2, SCP2, CACNA1C, TCRA, POLE, and FAM3D, were associated with narcolepsy. Some of these associations were further supported by gene expression analyses and an association study in essential hypersomnia (EHS), CNS hypersonia similar to narcolepsy.