The mouse four-jointed box protein 1 (fjx1) gene and Drosophila four-jointed (fj) gene are homologues [, , , ]. Fj encodes a partially secreted transmembrane (TM) type II glyco- protein. In Drosophila, the gene is a downstream target of the Notch signalling pathway in leg-segmentation and planar-cell polarity processes during eye development. Conserved in vertebrates, the murine homologue, fjx1, is also a direct target of Notch signalling []. The gene is expressed in mouse brain, in the peripheral nervous system, in epithelial cells of multiple organs, and during limb development [].The protein product is processed and secreted as a presumptive ligand []. fjx1 binding sites have been identified at complementary locations, suggesting that fjx1 may function as a novel signalling molecule [].
