Total RNA was isolated from mouse testes using the mirVana™ miRNA Isolation Kit (Life Technologies). Total RNA samples were treated with Turbo DNase (Invitrogen) and RNA cleanup was done following the manufacturer's instructions using the RNeasy Mini Kit (Qiagen). Small RNA sequencing libraries were prepared as previously described (Li et al., 2009a). Briefly, total RNA was isolated from flash frozen koala or mouse tissue using the mirVana miRNA Isolation Kit (Ambion/Life Technologies). Small RNAs were sized selected and purified from a 15% denaturing polyacrylamide-urea gel using the ZR small-RNA™ PAGE Recovery Kit (Zymo Research).