Despite an ongoing debate whether frequency of CD271+ human tumor-initiating cells is over- or under-estimated as a result of modifications in the mouse xenotransplantation protocols, including the use of high-protein matrigel12,13, their true physiological frequency in human patients cannot be determined due to the fact that it would require isogenic transplantations, which are impossible to perform. The gene discussed is NGFR; the disease is neoplasm.