In 1923, the concept of the reticuloendothelial system was introduced, distinguishing macrophages from other small phagocytic cells, such as neutrophils.11 By the 1970s, scientists had discovered the clonal stimulating factors that induce macrophage differentiation, with colony stimulating factor-1 (CSF-1) being the first such factor discovered.12 In 1972, research revealed the prevailing theory that the immune system may promote cancer growth.13 The development of monoclonal antibody technology in the 1980s made it possible to identify and isolate macrophages. The gene discussed is CSF1; the disease is cancer.