Primary 4T1 mammary tumours can induce the production of CCL17/TARC (thymus- and activation-regulated chemokine) in the pre-metastatic lung, which guides the recruitment of CCR4+ TREG cells and cancer cells to this site.94 The TREG cells then protect cancer cells by inhibiting NK cells, thereby facilitating metastasis formation. The gene discussed is CCR4; the disease is cancer.