In tumours, CCL2 can be secreted by multiple cell types, including tumour cells [18–21], myeloid-derived suppressor cells (MDSCs) [22], mesenchymal stem cells (MSCs) [23], tumour-associated macrophages (TAMs) [19, 24–26], tumour-associated neutrophils (TANs) [27], and cancer-associated fibroblasts (CAFs) [28–30]. The gene discussed is CCL2; the disease is neoplasm.