Brain tumor
Glioblastoma multiforme (GBM) is the most aggressive (grade IV) and most common form of a malignant brain tumor. Even when aggressive multimodality therapy consisting of radiotherapy, chemotherapy, and surgical excision is used, median survival is only 12–17 months. Standard therapy for glioblastoma multiforme consists of maximal surgical resection of the tumor, followed by radiotherapy between two and four weeks after the surgical procedure to remove the cancer, then by chemotherapy, such as temozolomide. Most patients with glioblastoma take a corticosteroid, typically dexamethasone, during their illness to relieve symptoms. Experimental treatments include targeted therapy, gamma knife radiosurgery, boron neutron capture therapy and gene therapy.
Merkel-cell carcinoma
MCV likely contributes to the development of the majority of MCC. About 80% of MCC tumors are infected with MCV, with the virus integrated in a monoclonal pattern, indicating that the infection was present in a precursor cell before it became cancerous. MCV, a polyomavirus, is the first polyomavirus strongly suspected to cause tumors in humans. MCV is ubiquitous and is thought to be part of the human skin microbiome. Intriguingly, most MCV viruses obtained so far from tumors have specific mutations that render the virus uninfectious.
MCC patients whose tumors contain MCV have higher antibody levels against the virus than similarly infected healthy adults. A study of a large patient registry from Finland suggests that individuals with MCV-positive MCC's have better prognoses than do MCC patients without MCV infection.
Like other tumor viruses, most people who are infected with MCV do not develop MCC. 
As of 2008, it was unknown what other steps or co-factors were required for MCC-type cancers to develop.