BMP4 and cancer: Anemia or hypoxia accompanied by inflammation, which occur frequently during cancer development (77, 78), chronic infection (79), severe trauma (80), and chronic psychological stress (81, 82), disrupt the homeostasis between erythroid cell production through steady-state erythropoiesis and clearance of senescent or damaged erythroid cells by phagocytes, inducing stress erythropoiesis (79, 83); this process is regulated by bone morphogenetic protein 4 (BMP4), SCF, Hedgehog, EPO, growth-differentiation factor 15 (Gdf15), and glucocorticoids (GCs) (Figure 2) (84).