Simple combinations of a few secondary structure elements with a specific geometric arrangement have been found to occur frequently in protein structures. These units have been called either supersecondary structure or motifs.Motifs are formed by packing side Chains from adjacent α helices or β strands close to each other. Simple motifs combine to form complex rnotifs. Several motifs usually combine to form compact globular structures,Which are called domains.In other words,the cores of domains are built up from combinations of small motifs，such as α-loop-α, α-loop-β, β-loop-α，β-loop-β or β-α-β motifs.