1. RNA Biology Laboratory, RIKEN Advanced Science Institute, 2-1 Hirosawa, Saitama 351-0198, Japan
2. PRESTO, Japan Science and Technology Agency, Kawaguchi-shi, Saitama 332-0012, Japan
3. Functional RNomics Team, Biomedicinal Information Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 2-4-7 Aomi, Koutou, Tokyo 135-0064, Japan
4. Laboratory for Animal Resources and Genetic Engineering, RIKEN Center for Developmental Biology, 2-2-3 Minatojima Minami, Chuou-ku, Kobe 650-0047, Japan