SPDL1 and central nervous system cancer: Most studies have found sPD-L1 to be elevated in cancer patients compared to healthy donors, with higher levels observed in patients with non-small cell lung cancer (NSCLC) [44–47], small cell lung cancer [48], gastric cancer [49, 50], hepatocellular carcinoma [51–53], colorectal cancer [54], nasopharyngeal carcinoma [55], differentiated thyroid carcinoma [56], glioma [22], basal cell carcinoma [57], renal cell carcinoma [58], prostate cancer [59], and ovarian cancer [23, 60, 61].