Immunoglobulin superfamily 11 (IgSF11) overexpression has been observed in several cancer types, including gastric cancer, hepatocellular carcinoma, colorectal cancers and gliomas, and linked to aggressive clinicopathological features. Therefore, IgSF11 has been proposed as a novel immune checkpoint ligand and a candidate for cancer immunotherapy. However, the potential role and clinical significance of IgSF11 in human breast cancer remain unexplored.