ACKR2 and neoplasm: Based on a large amount of evidence from previous studies, they proposed a hypothesis that patients with increased expression levels of atypical chemokine receptor 2 (ACKR2) receptors to which CCL2 binds have less chance for tumor metastasis, whereas patients with decreased ACKR2 expression but high levels of CCR2+ monocytes are more likely to develop metastasis and have worse outcomes.