Acts as a suppressor of microRNA (miRNA) biogenesis by specifically binding the precursor let-7 (pre-let-7), a miRNA precursor. Acts by binding pre-let-7 and recruiting ZCCHC11/TUT4 uridylyltransferase, leading to the terminal uridylation of pre- let-7. Uridylated pre-let-7 miRNAs fail to be processed by Dicer and undergo degradation. Specifically recognizes the 5'-GGAG-3' motif in the terminal loop of pre-let-7. Also recognizes and binds non pre-let-7 pre-miRNAs that contain the 5'-GGAG-3' motif in the terminal loop, leading to their terminal uridylation and subsequent degradation. Mediates MYC-mediated let-7 repression. Isoform 1, when overexpressed, stimulates growth of the breast adenocarcinoma cell line MCF-7. Isoform 2 has no effect on cell growth. Interacts with ZCCHC11/TUT4. Might be negatively regulated by the microRNA let-7b. High expression in testis, fetal liver, placenta and in hepatocellular carcinoma (HCC). Isoform 1 is only detected in moderately and poorly differentiated HCC tissues and placenta. Isoform 2 is detected in fetal liver, non-tumor liver tissues, as well as well-differentiated tumor tissues. Belongs to the lin-28 family. 2 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.