CCL2 and neoplasm: The CSF‐1 mainly regulates the differentiation of yolk sac‐derived and bone‐marrow‐derived macrophages,[43] while IL‐34 prefers to regulate the development of microglia and Langerhans cells.[34, 37, 49] Therefore, CSF‐1 is an important attractant for monocytes recruitment.[34, 35, 43] In addition, the previously mentioned CCL2 is also a crucial factor for monocyte recruitment.[43] After being recruited into the tumor site, many factors can facilitate monocytes to differentiate into tumor‐supportive M2 phenotype to promote tumor progression.[43]