Human organ cultures can be used to study stem cells within their niche. For example, in human cardiac tissue affected by ischemic heart disease, resident stem cells, identified as small round cells that express CD133 but not CD45, express TNF receptor 2 (TNFR2) and respond to TNF, in a manner dependent upon TNFR2 signaling, by entering cell cycle and expressing characteristic markers of mature cardiomyocytes (7). The gene discussed is TNFRSF1B; the disease is coronary artery disorder.