Despite the fact that the identified genes were relatively cell linespecific, induction of some EGF pathway-related transcripts in the analyzed cell lines suggests that this molecular pathway may be involved in the tumor cell response to cycling hypoxia. Conceivably, they may mediate cycling hypoxia-induced tumor aggressiveness, though, due to the lack of established models of cycling hypoxia, caution in data interpretation is recommended. Nevertheless, these genes as well as newly reported more universal hypoxia-responsive genes are worth further validation. There is increasing evidence that tumors are hierarchically organized by heterogeneous populations including a small fraction of cancer stem cells. CSC share many similarities with normal stem cells, such as self-renewing capacity and multilineage differentiation properties. In addition, CSC are highly tumorigenic and can generate phenocopies of the primary human malignancy in immunocompromised mice. From a clinical point of view, CSC are responsible for tumor maintenance, sustentation, recurrence and resistance to conventional treatments. A CSC fraction has been isolated in many cancers, including glioma, using various approaches. Most glioma CSC have been derived from clinical tumor specimens while only a few have been derived from established cell lines: Rat C6 cells and human malignant glioma cell lines have been used. Some Authors do not recommend cell lines as a source of CSC because they grow in serum containing medium, which gives rise to cells that differ genetically and biologically from those of the primary tumors from which they were derived. Nevertheless, cancer cell lines have some advantages with respect to tumor tissue. Indeed, they do not present any contaminating normal stem cells, can be considered a homogeneous sample and it is easy to obtain large amounts of them. Therefore, identification and characterization of CSC from established cell lines may provide important tools for exploring the biology of CSC. No single marker has been shown to be sufficient to confer stem-cell-like properties, thus a combination of different markers is used to identify and isolate CSC in glioma, including Nestin, Sox2 and Musashi-1. These molecules are expressed at high levels in neural stem cells and are frequently considered a hallmark of the undifferentiated state. When exposed to fetal bovine serum, CSC differentiate down the lineage of the parental tumor. Therefore, CSC derived from VE-822 gliomas preferentially differentiate to astrocytes.