GDF11 and glioblastoma: Examples include growth differentiating factor 11 (GDF11) and Activin βB (ACTβB), which negatively regulate self-renewal rates in progenitor and stem cells in the olfactory epithelium of mice [7,8]; transforming growth factor beta (TGF-β) [9], which is mutated in a variety of tumors [10–12]; the bone morphogenetic protein 4 pathway (BMP4) that is inactivated in glioblastomas [13]; and the adenomatous polyposis coli (APC) tumor suppressor gene that is inactivated in colorectal cancer, with concomitant activation of the Wnt cascade [14].