LRR-containing proteins are present in over 2000 proteins from viruses to eukaryotes. Most LRRs are 20–30 amino acids long, and the repeat number ranges from 2 to 42. The known structures of 14 LRR proteins, each containing 4–17 repeats, have revealed that the LRR domains fold into a horseshoe (or arc) shape with a parallel β-sheet on the concave face and with various secondary structures, including α-helix, 310-helix, and pII helix on the convex face. We developed simple methods to charactere quantitatively the arc shape of LRR and then applied them to all known LRR proteins. A quantity of 2Rsin(φ/2), in which R and φ are the radii of the LRR arc and the rotation angle about the central axis per repeating unit, respectively, is highly conserved in all the LRR proteins regardless of a large variety of repeat number and the radius of the LRR arc. The radii of the LRR arc with β-α structural units are smaller than those with β-310 or β-pII units. The concave face of the LRR β-sheet forms a surface analogous to a part of a Möbius strip. Proteins 2004;54:000–000. © 2003 Wiley-Liss, Inc.