RHEB and squamous cell carcinoma: On the other hand, ATF6α prolongs survival of dormant tumor cells, but not proliferative squamous carcinoma cells, through transactivation of the Ras homolog enriched in brain (Rheb; a critical activator of the mammalian target of rapamycin [mTOR]) and thus activation of mTOR signaling (Schewe and Aguirre-Ghiso 2008).