Fibin is a family of eukaryotic proteins expressed in the lateral plate mesoderm of presumptive pectoral fin bud regions. It acts as a signal molecule for the expression of Tbx5, a gene involved in the specification of fore-limb identity. Fibin is found to be expressed in cerebellum, skeletal muscle and many other embryonic as well as adult mouse tissues, suggesting roles in both embryogenesis and in adult life. Although Fibin is routed through the endoplasmic reticulum (ER) no significant evidence for secretion is found. Fibin is post-translationally modified and forms dimers when expressed heterologously and its expression is regulated by a number of cellular signalling pathways.