1. Department of Molecular Biology, Umeå University , SE-901 85 Umeå , Sweden
2. Wallenberg Centre for Molecular Medicine, Umeå University , SE-901 85 Umeå , Sweden
3. Laboratory of RNA Regulatory Networks, Department of Cellular, Computational and Integrative Biology (CIBIO), University of Trento , Via Sommarive 9, 38123 Trento (TN), Italy