Total RNA (600 ng) was ligated to 5 μM of 5′-adenylated, 3′-blocked adaptor (Universal miRNA Cloning Linker, New England BioLabs) with 280 units of T4 RNA ligase, Truncated KQ (New England BioLabs), 25% PEG 8000 and 1 μl of RNaseOUT (Life Technologies) in a 20-μl reaction at 25 °C for 16–24 h. After cleanup with RNA Clean and Concentrator columns (Zymo Research), followed by DNase treatment, cDNA was synthesized with 5 pmol of universal RT primer (Supplementary Table 6) and SuperScript III reverse transcriptase. PCR amplification was carried out using 5 μM of the TERC_L2 and universal RT or TERC_L3 and universal RT primer sets (Supplementary Table 6) with SsoAdvanced Universal SYBR Green Supermix (Bio-Rad). PCR products were directly analyzed on 2.5% agarose gels to visualize mature TERC and extended TERC transcripts or subjected to QIAquick PCR purification columns (Qiagen) for library preparation for deep sequencing. For Sanger sequencing, 3′ RACE PCR products were directly cloned into the pCR4_TOPO vector (Life Technologies), and individual clones were sequenced using the TERC_L2 or TERC_L3 primer.