Through the use of mutation breeding, we developed the cultivar LGC1, which has a low glutelin and high prolamine content, from Nihonmasari. Because of the indigestibility of prolamine, LGC1 can be used as a low-protein rice in practice. It is thus a prospective food for people such as patients with kidney disease who must restrict their protein intake. Furthermore, to reduce the content of easy-to-digest protein in LGC1, we developed two new rice cultivars, ‘LGC-Katsu’ and ‘LGC-Jun’, from a cross between LGC1 and a mutant line of Koshihikari (induced by gamma-irradiation) that was deficient in 26-kDa globulin (another easy-to-digest protein). The glutelin content of these two new cultivars is reduced to about one-third that of regular cultivars, and 26-kDa globulin is completely absent. Consequently, the total amount of easy-to-digest protein in the new cultivars is about half that of regular cultivars, and about 15% less than in LGC1. These cultivars should therefore greatly help in the dietary management of patients with chronic renal failure. We also discussed the genome structures and mechanisms of the seed protein composition mutation.