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 primary function of these motifs is to provide a versatile structural framework to mediate the formation of protein-protein interactions. LRRs are present in a variety of proteins with diverse structure and function, including innate immunity and nervous system development. Several human diseases are associated with mutations in genes encoding LRR-containing proteins. The leucine-rich repeat-containing protein 21 (LRRC21), also designated leucine-rich repeat, immunoglobulin-like domain and transmembrane domain-containing protein 1 (LRIT1) or PAL, is a 623 amino acid protein that contains 6 LRR repeats. LRRC21 is expressed in retina and may play a role in phototransduction.