Model proposed in the study by Zhao et al. Physiological activation of p38MAPK/MK2 during circadian cycles causes differential ROS levels and HIF-1α–BMAL1-dependent CXCR4 regulation, explaining opposite trafficking patterns of mouse and human leukocytes in chimeric immunodeficient mice. hWBC, human white blood cell; mWBC, mouse white blood cell; P, phosphorylation; WBC, white blood cell.