The 2007 WHO Classification of Tumors of the Central Nervous System was the last classification mainly based on microscopy features. The new 2016 WHO Classification of Tumors of the Central Nervous System was a paradigm shift: some of the tumors were defined also by their genetic composition as well as their cell morphology. The grading of gliomas changed importantly and glioblastoma was now mainly classified according to the status of isocitrate dehydrogenase (IDH) mutation: IDH-wildtype or IDH-mutant. Molecular alterations
Four subtypes of glioblastoma have been identified based on gene expression:
Classical: Around 97% of tumors in this subtype carry extra copies of the epidermal growth factor receptor (EGFR) gene, and most have higher than normal expression of EGFR, whereas the gene TP53 (p53), which is often mutated in glioblastoma, is rarely mutated in this subtype. Loss of heterozygosity  in chromosome 10 is also frequently seen in the classical subtype alongside chromosome 7 amplification. The proneural subtype often has high rates of alterations in TP53 (p53), and in PDGFRA, the gene encoding a-type platelet-derived growth factor receptor, and in IDH1, the gene encoding isocitrate dehydrogenase-1. The mesenchymal subtype is characterized by high rates of mutations or other alterations in NF1, the gene encoding neurofibromin 1 and fewer alterations in the EGFR gene and less expression of EGFR than other types.
The average age at diagnosis is 64, and the disease occurs more commonly in males than females. Signs and symptoms
Common symptoms include seizures, headaches, nausea and vomiting, memory loss, changes to personality, mood or concentration, and localized neurological problems. The kind of symptoms produced depends more on the location of the tumor than on its pathological properties. The tumor can start producing symptoms quickly, but occasionally is an asymptomatic condition until it reaches an enormous size. Risk factors
The cause of most cases is unclear. About 5% develop from another type of brain tumor known as a low-grade astrocytoma. Genetics
Uncommon risk factors include genetic disorders such as neurofibromatosis, Li–Fraumeni syndrome, tuberous sclerosis, or Turcot syndrome. Previous radiation therapy is also a risk. For unknown reasons, it occurs more commonly in males. Environmental
Other associations include exposure to smoking, pesticides, and working in petroleum refining or rubber manufacturing.Glioblastoma has been associated with the viruses SV40, HHV-6, and cytomegalovirus. Other
Research has been done to see if consumption of cured meat is a risk factor. No risk had been confirmed as of 2013. Similarly, exposure to radiation during medical imaging, formaldehyde, and residential electromagnetic fields, such as from cell phones and electrical wiring within homes, have been studied as risk factors. As of 2015, they had not been shown to cause GBM. Pathogenesis
The cellular origin of glioblastoma is unknown. Because of the similarities in immunostaining of glial cells and glioblastoma,  gliomas such as glioblastoma have long been assumed to originate from glial-type cells.