Tumor-suppressor let-7 pre-microRNAs (miRNAs) are regulated by terminal uridylyltransferases TUT7 and TUT4 that either promote let-7 maturation by adding a single uridine nucleotide to the pre-miRNA 3′ end or mark them for degradation by the addition of multiple uridines. The gene discussed is TUT4; the disease is neoplasm.