We use conditional mouse models of active YAP1 expression and LATS1/2 deletion and show that suppression of YAP1 activity in NEX/NeuroD6 expressing neuronal precursor cells is essential for limiting proliferation and enabling neuronal differentiation in the brain, whereas activation of YAP1 is sufficient to produce tumours that display features similar to human ependymoma. The gene discussed is NEUROD6; the disease is neoplasm.