CD9 and digestive system cancer: The outcomes of subgroup analysis revealed that the prognostic significance of CD9 expression changed with respect to the cancer type (head and neck cancer and leukemia/lymphoma), CD9 expression detection method (flow cytometry), publication year (after 2005), and sample size (> 110) for OS, as well as cancer type (breast cancer and digestive system cancer), publication year (after 2005), and race (Asian) for DFS.