1. Silk-Materials Research Unit, National Institute of Agrobiological Sciences, 1-2 Ohwashi, Tsukuba, Ibaraki 305-8634, Japan
2. Applied Cell Biotechnologies, Inc., 1-1-48 Suehirocho, Tsurumi-ku, Yokohama, Kanagawa 230-0045, Japan
3. Department of Laboratory Medicine, The Jikei University School of Medicine, 3-25-8 Nishi-shinbashi, Minato-ku, Tokyo 105-8461, Japan
4. Division of Biological Chemistry and Biologicals, National Institute of Health Sciences, 1-18-1 Kamiyoga, Setagaya-ku, Tokyo 158-8501, Japan