Microglia maintain brain health and play important roles in disease and injury. Despite their known ability to proliferate, the precise nature of the population(s) capable of generating new microglia in the adult brain remains controversial. We identified Prominin-1 (CD133 or Prom1) as a putative cell surface marker of committed brain myeloid progenitor cells. We demonstrate that Prom1 expressing cells isolated from mixed cortical cultures will generate new microglia in vitro . To determine whether Prom1 expressing cells generate new microglia in vivo , we used tamoxifen inducible fate mapping in male and female mice. Induction of Cre recombinase activity at 10 weeks in Prom1 expressing cells lead to the expression of TdTomato in all Prom1 expressing progenitors and newly generated daughter cells. We observed a population of new TdTomato expressing microglia at six months of age that increased in size at nine months. When microglia proliferation was induced using a transient ischemia/reperfusion paradigm, ...