ESR1 and breast cancer: Christgen et al., (2009) reported high frequency of KAI-1 positive cases in both ER-negative primary tumors and ER negative metastases, which is paradoxical to the notion that KAI-1 suppresses metastasis and stated that KAI-1 may not be a useful marker to investigate invasive/ metastatic potential in breast cancer (Christgen et al., 2009).