While it has been established that cancer stem cells (CSCs) are important for the phenotypic heterogeneity of glioblastoma, allowing tumor progression, it has been unclear if CSCs are a stable entity (clonal subpopulation with unique properties), or a reversible cellular state that can be acquired by any cancer cell. A new study performed in Luxembourg and Germany suggests that phenotypic heterogeneity is a consequence of their intrinsic plasticity.