B2M and breast cancer: Some studies have shown that metastatic breast cancer is likely associated with a β2M/creatinine ratio > 3.8 [16], and patients with downregulated β2M show significantly improved overall survival compared to patients with lymph node-positive breast cancer with normal β2M levels [17], and β2M overexpression could drive EMT and promote the growth, invasion, and metastasis of human breast cancer cells [14].