Total RNA was extracted using Trireagent (Sigma-Aldrich). The total RNA was then processed further according to the Small RNA Expression Kit (Applied Biosystems). In short, the quality of the total RNA was determined using Agilent Bioanalyzer, and 10 μg total RNA with a RNA integrity number (RIN) of 9.7 or higher was depleted of ribosomal RNA using the Ribominus kit according to the protocol (Invitrogen). RNase III (Ambion) was used for RNA fragmentation. Further RNA-Seq library construction was performed according to the Small RNA Expression Kit protocol (Applied Biosystems) using 200 ng of fragmented RNA as input.