OsBEIIb plays a specific role in the transfer of short chains, which might then be extended by starch synthase to form the A and B1 chains of amylopectin cluster in rice endosperm. Mutation in the BEIIb gene specifically alters the structure of amylopectin in the endosperm by reducing short chains with degree of polymerization of 17 or less, with the greatest decrease in chains with degree of polymerization of 8 to 12 (Nishi et al., 2001). Complementation experiment of OsBEIIb gene into a BEIIb-defective mutant shows fine structure of amylopectins in endosperm. Overexpression of BEIIb lines shows the accumulation of excessive branched, water-soluble polysaccharides instead of amylopectin (Tanaka et al., 2004).