FJX1 is a 437 amino acid protein that belongs to the FJX1/FJ family. FJX1 is highly conserved in vertebrates and is expressed in the peripheral nervous system, epithelial cells of multiple organs and during limb development. FJX1 is processed and secreted as a presumptive ligand and may act as an inhibitor of dendrite extension and branching. In Drosophila, FJX1 is important for growth and differentiation of legs and wings, and for proper development of the eyes. FAT4, an essential gene that has a key role in vertebrate PCP (planar cell polarity), represses FJX1 expression which may lead to cystic diseases in humans.