CD44 and breast cancer: In 2006, CD44-positive cells isolated from human breast cancer cells, known as mammospheres, showed both radiation resistance and low levels of reactive oxygen species (ROS).42 In 2009, studies using MMTV-Wnt-1 mice demonstrated that CD44-positive CSCs have reduced ROS levels owing to the increased expression of antioxidant defense genes, including those involved in GSH synthesis.43 These findings suggest that CD44-positive CSCs protect themselves from oxidative stress by enhancing their antioxidant capacities.