RNA purification and fragmentation analysis. RNAs were extracted by TRIZOL (Invitrogen, Carlsbad, CA) following the manufacturer's protocol and their concentrations were measured using a NanoDrop reader (ThermoFisher, Wilmington, DE). Fragmentation of RNA was assessed by denaturing agarose gel electrophoresis (Li and Pestka, 2008) or capillary electrophoresis using an Agilent 2100 Bioanalyzer with a NanoChip (Agilent, Santa Clara, CA) according to manufacturer's instructions.