PTEN and cancer: Several intracellular signaling pathways are critical for normal stem cell function: the core pathways are the phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT), phosphatase and tensin homolog (PTEN), WNT, and Notch pathways; these promote cell proliferation and, thus, the formation of cancer stem cell-like colonies (Table 2).