MYC and glioblastoma: PKM2 is selectively expressed at low levels in GBM but is completely absent in normal brain [34]. c-Myc, which is also targeted by let-7a, upregulates the expression of the heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1) splicing factor which, in turn, downregulates let-7a in a positive feedback loop [68].