MTOR and cancer: Technically, MM cancer stem cells (CSCs) have been identified as a “CD138- side population (SP)” in flow cytometric analysis of MM cell lines and MM primary cells, with the most important stem cell-related markers and pathways including ALDH (aldehyde dehydrogenase), Hedgehog, Wnt, Notch and PI3K/Akt/mTOR being activated in MM SP cells [43, 76].