The leucine-rich (LRR) repeat is a 20-30 amino acid motif that forms a hydrophobic a/∫ horseshoe fold, allowing it to accommodate several leucine residues within a tightly packed core. All LRR repeats contain a variable segment and a highly conserved segment, the latter of which accounts for 11 or 12 residues of the entire LRR motif. The LRRTM protein family plays a role in the regulation of various cellular events during nervous system development. Localizing predominantly to the nervous system, LRRTM family members are known to exhibit synaptogenic activity. LRRTM4 (leucine-rich repeat transmembrane neuronal protein 4) is a 590 amino acid member of the LRRTM protein family. Expressed in neuronal tissues, LRRTM4 may play a role in the development and maintenance of the vertebrate nervous system. A single-pass type I membrane protein, LRRTM4 contains 10 LRR repeats. LRRTM4 is expressed as two isoforms produced by alternative splicing.