Ithimakin et al. suggested that this unexpected finding might be explained by the cancer stem cell (CSC) hypothesis38 according to which CSCs are implicated in treatment resistance39,40 and tumour recurrence.41 Specifically, Ithimakin et al. demonstrated that, in some tumours with moderate HER2 levels, HER2 is selectively expressed in the CSC population. The gene discussed is ERBB2; the disease is neoplasm.