HEATR1 and glioblastoma: Indeed, blocking ribogenesis by treatment with low dosage of ActD that specifically inhibits rRNA polymerase (Pol) I transcription but not Pol II or III activity (Boulon et al, 2010) results in dramatic reduction in EU incorporation in nuclei of GBM cells comparable to that upon HEATR1 inhibition (Fig. 5C–E), and abolishes tumourigenic potential in GSCs as measured via soft agar assays (Appendix Fig. S5A–F, compare to Fig. 3M–T).