HHLA2 and cholangiocarcinoma: Several studies have demonstrated that overexpression of HHLA2 in tumor cells is linked to adverse clinical outcomes and reduced survival rates in patients with various types of cancer, such as prostate cancer [12], hepatocellular carcinoma [13], lung adenocarcinoma [10], gastric cancer [14], bladder urothelial carcinoma [15], cholangiocarcinoma [16], colorectal carcinoma [17], osteosarcoma [18], and breast cancer [19].