The leucine-rich (LRR) repeat is a 20-30 amino acid motif that forms a hydrophobic ?/∫ 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. LRRC54 (leucine-rich repeat-containing protein 54), also known as tsukushin, TSKU or E2-induced gene 4 protein (E2IG4), is a 353 amino acid secreted protein that likely localizes to the cell membrane and extracellular compartments. Involved in extracellular secretion and intracellular transport, LRRC54 can be induced by 17-beta-estradiol. Containing nine LRR repeat and a cleavable signal peptide, the gene encoding LRRC54 maps to human chromosome 11, which houses over 1,400 genes and comprises nearly 4% of the human genome. Jervell and Lange-Nielsen syndrome, Jacobsen syndrome, Niemann-Pick disease, hereditary angioedema and Smith-Lemli-Opitz syndrome are associated with defects in genes that maps to chromosome 11.