We previously showed that, in this transgenic mouse line, GPR17+-OPCs (i.e., GFP+ cells) are reluctant to differentiate under healthy conditions, but can rapidly respond to injury, suggesting they may represent a ‘reserve pool’ of adult progenitors maintained for repair purposes.10 Here, we provide the first comprehensive in vivo fate-mapping analysis of these cells in a model of permanent brain ischemia up to 8 weeks. The gene discussed is GPR17; the disease is brain ischemia.