library_construction_protocol: The heart of mice was removed and flash frozen by liquid nitrogen. After suspending in lysis buffer, nuclear fraction, insoluble debris and mitochondria were removed by centrifugation. Extracts were digested with RNase I (Ambion). Then, monosomes were purified by ultra-centrifugation (Beckman coulter, Optima MAX-TL Ultracentrifuge) through 50% sucrose. Next, ribosome pellets were digested with Proteinase K. Finally, the ribosome protected fragments (RPFs) were size-selected (25-35 nt) by polyacrylamide gel electrophoresis, and converted to Illumina sequencing library using NEBNext Small RNA Library Prep Set for Illumina (New England Biolabs). RNA libraries were prepared for sequencing using standard Illumina protocols.