library_construction_protocol: Total RNAs were isolated using TRIzol (Thermo Fisher Scientific) and treated with DNase I (Takara Bio). Sequencing libraries were generated as previously described (Kim et al. 2019). In brief, we mixed 20 µg of total RNAs with 10 nmole of 30 equimolar spike-in RNAs described in our previous study (Kim et al. 2019). Small RNAs were enriched by size fractionation by 15% urea-polyacrylamide gel electrophoresis and ligated to the randomized adaptor at the 3′ and 5′ ends. The ligated RNAs were reverse-transcribed using SuperScript IV Reverse Transcriptase (Thermo Fisher Scientific), amplified using Phusion High-Fidelity DNA Polymerase (Thermo Fisher Scientific), and subjected to high-throughput sequencing on the MiSeq platform (Illumina).