PTEN and neoplasm: Regarding the tumor suppressors, beyond the fact that rRNA transcription may be stimulated in some types of cancer by deletion or mutation of PTEN which normally represses Pol I transcription by disrupting the SL1 complex [28], there is evidence that the usual increase of ribosome biogenesis in cancer cells is the consequence of the very frequent alterations of the status of the two major tumor suppressors, TP53 and RB.