*4.2. Lentivirus Vector Construction for Overexpression of LIN28A and LIN28B*

To overexpress LIN28A and LIN28B, pCDH lentiviral expression vector (System Biosciences, Palo Alto, CA, USA) was used. The mRNA was extracted from day 16 TE using RNeasy Mini Kit (Qiagen Inc. Germantown, MD, USA) following the manufacturer's protocol, and then reverse transcribed to cDNA using iScript cDNA synthesis kit (Bio-Rad Laboratories, Hercules, CA, USA). The cDNA was amplified using PCR primers for LIN28A or LIN28B (Supplementary Table S2). The PCR primers included restriction sites for NheI and SwaI restriction enzymes. The resulting PCR amplicons were gel purified and cloned into the StrataClone PCR cloning vector using StrataClone PCR Cloning KIT (Agilent, Santa Clara, CA). StrataClone vector with successful cloning of PCR product was double digested using NheI/SwaI. The double digested product was cloned in double digested pCDH vector and confirmed by sanger sequencing.
