SPP1 and glioblastoma: The strongly increased expression and secretion of OPN in the tumour microenvironment results in increased levels of circulating OPN (cOPN) in the peripheral blood of patients with various types of cancer, including mesothelioma, glioblastoma, hepatocellular carcinoma, breast, bone and head and neck cancers [38,39,40,41,42,43,44,45], which in some cases has been shown to be prognostically relevant.