CD44 and breast cancer: In 2003, studies by Clarke and colleagues showed that breast tumor initiating stem cells (CD44+CD24−/low lineage subpopulation) isolated from primary breast cancers could form tumors when transplanted into non-obese diabetic/severe combined immunodeficient (NOD/SCID) immunocompromised mice (Al-Hajj et al., 2003), however the remaining populations formed no detectable tumors even 29 weeks after injection into mice.