Recent advances in the field of single-cell RNA sequencing together with insights from fate mapping studies, show that, upon onset of arthritis, blood-derived CCR2+ monocytes start to infiltrate the synovial tissue and give rise to different populations of monocyte-derived macrophages that are characterized by a pro-inflammatory activation profile (including high expression levels of IL-1β). The gene discussed is CCR2; the disease is Arthritis.