Advances in genetic research have facilitated the identification of genes responsible for semidwarfism in rice. The incomplete recessive gene d47 is responsible for the semidwarfism of DGWG, the parent line of IR8 [8–10]. Later, the incomplete recessive gene sd1 in Calrose 76 was shown to be allelic to d47 [4, 11, 12]. Moreover, allelic relationships of the semidwarf gene have been found in Taichung Native 1, derived from DGWG; Shiranui, derived from Jukkoku; and d49 in the mutant cultivar Reimei [13, 14]. Finally, all these semidwarf cultivars carry alleles at the sd1 locus, despite their different parentage [14–18]. The sd1 alleles, on the long arm of chromosome 1, encode loss-of-function mutations in GA20-oxidase (OsGA20ox2), which regulates the synthesis of biologically active gibberellins (GAs), which catalyze three steps in the GA biosynthesis pathway [15–18]. Taken together, a single semidwarf gene, sd1, solely confers the semidwarf phenotype of cultivars commonly grown around the world due to its nondetrimental effects on grain yield .