Differentiation of neural stem cells (NSCs) to neurons requires the activation of genes controlled by the repressor element
1 (RE1) silencing transcription factor (REST)/neuron-restrictive silencer factor (NRSF) protein complex. Important components
of REST/NRSF are phosphatases (termed RNA polymerase II C-terminal domain small phosphatases [CTDSPs]) that inhibit RNA polymerase
II and suppress neuronal gene expression in NSCs. Activation of genes controlled by CTDSPs is required for neurogenesis, but
how this is achieved is not fully understood. Here we show that ctdsp2 is a target of miR-26b, a microRNA that is encoded in an intron of the ctdsp2 primary transcript. This intrinsic negative feedback loop is inactive in NSCs because miR-26b biogenesis is inhibited at
the precursor level. Generation of mature miR-26b is activated during neurogenesis, where it suppresses Ctdsp2 protein expression
and is required for neuronal cell differentiation in vivo.