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. This evidence concerns the gene CD9 and breast cancer.