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Simple Summary: Glioblastoma is one of the worst cancer types and the most common cancer originating within the brain. Patients afflicted by glioblastoma suffer from poor prognosis, a lack of specific therapies and frequent tumor recurrences. Many researchers are confident that glioblastoma cells can display traits of stem cells and that these attributes lead to an aggressive growth and high rate of recurrence. Based on our previous work that demonstrates that the "sun hormone" vitamin D3 can block these stem cell traits, we have now gained additional insights into the effects of the active form of vitamin D3, calcitriol. We can show that specific gene variants of the vitamin D3 receptor might be responsible for the sensitivity towards calcitriol and that sensitive cells are blocked in their stemness attributes as well as migratory potential. Glioblastoma is the most common primary brain cancer in adults and represents one of the worst cancer diagnoses for patients. Suffering from a poor prognosis and limited treatment options, tumor recurrences are virtually inevitable. Additionally, treatment resistance is very common for this disease and worsens the prognosis. These and other factors are hypothesized to be largely due to the fact that glioblastoma cells are known to be able to obtain stem-like traits, thereby driving these phenotypes. Recently, we have shown that the in vitro and ex vivo treatment of glioblastoma stem-like cells with the hormonally active form of vitamin D3, calcitriol (1α,25(OH)2-vitamin D3) can block stemness in a subset of cell lines and reduce tumor growth. Here, we expanded our cell panel to over 40 different cultures and can show that, while half of the tested cell lines are sensitive, a quarter can be classified as high responders. Using genetic and proteomic analysis, we further determined that treatment success can be partially explained by specific polymorphism of the vitamin D3 receptor and that high responders display a proteome suggestive of blockade of stemness, as well as migratory potential. [ABSTRACT FROM AUTHOR]