*4.1. Lentivirus Vector Construction for shRNA Expression*

Lentiviral infection was used to stably integrate and express shRNA targeting LIN28A or LIN28B mRNA in the host cell. Lentiviral vectors were constructed using the protocol previously described by Baker et al. [61]. Briefly, LIN28A targeting shRNA, LIN28B targeting shRNA, or scrambled control shRNA sequence (Table S1) were first cloned into the pLKO.1 vector (plasmid 10878, Addgene, Cambridge, MA, USA), which contained the human U6 promoter upstream of cloning site for shRNA cassettes. The human U6 promoter and downstream LIN28A/LIN28B/SC shRNA sequence within pLKO.1 was PCR amplified using a forward primer with a 50 XbaI restriction site(50 -TCTAGATTCACCGAGGGCCTATTTCCC-30 ) and a reverse primer containing a 30 XhoI restriction site (50 -GAATACTGCCATTTGTCTCGAGGTCG-30 ). The resulting PCR amplicon was gel purified and cloned into the StrataClone PCR cloning vector using StrataClone PCR Cloning KIT (Agilent, Santa Clara, CA). The human U6 promoter and LIN28A/LIN28B/SC shRNA DNA fragment was digested from StrataClone PCR cloning vector using XbaI/XhoI restriction enzymes. Subsequently, the DNA fragment was ligated into the pLL3.7 vector also digested with XbaI/XhoI. Insertion of the human U6 promoter and LIN28A/LIN28B/SC shRNA sequence into pLL3.7 was verified by sanger sequencing.
