STIP1 and glioma: In line with this, the PrPC cell surface ligand STI1/HOP can be translocated to the cell surface and/or secreted outside the tumor cell, acting both as an autocrine and paracrine factor in promoting cell growth, proliferation, and migration of various tumor cells, encompassing ovarian [149,150], renal [151], and glioma (Erlich et al., 2007) [128] cancer cells.