**2. Human gliomas**

Gliomas represent 40% of all primary central nervous system (CNS) tumors diagnosed. Among them, glioblastomas (GBM) are the most malignant, with a very poor survival time of about 15 months for most patients diagnosed with this grade IV brain tumor (CBTRUS 2011). High grade gliomas are the most common primary brain tumors in adults, and their malignant nature ranks them as the fourth largest cause of cancer death (Niclou *et al.*, 2010). There are four tumor grades for gliomas: Grade I which is a non-malignant, fairly circumscribed astrocytoma that is rare and appears in young adults; Grade II which are a more common diffusely infiltrating astrocytoma; Grade III which is an anaplastic astrocytoma; and Grade IV which is a glioblastoma multiforme (Niclou *et al.*, 2010). Grades II-IV gliomas generally are found in the adult population, and often recur following current treatment options (including surgical resection, radiotherapy, and chemotherapy) (Niclou *et al.*, 2010).

Grading and identification criteria that can be used to provide information regarding tumor behavior are cell proliferation (cellularity and mitotic activity), nuclear atypia, neovascularization and the presence of necrosis and/or apoptotic regions (Gudinaviciene *et al*., 2004). Grade II gliomas (also referred to as diffuse astrocytomas) and grade III gliomas (also referred to as anaplastic astrocytomas) only differ based on their mitotic activity, and this difference accounts for a substantial decrease in the 5-year survival for patients, from 47% to 29% (from grade II to III, respectively) (CBTRUS 2011). Grade IV gliomas (GBM) are often characterized by the presence of large necrotic areas (Gudinaviciene *et al*., 2004) and generally have a 3% 5-year survival (CBTRUS 2011).
