CD44 and neoplasm: The CSC2 number would be related to the niche volume, nutriments and oxygen quantities (Ezashi et al., 2005; Mohyeldin et al., 2010; Quail et al., 2012): in mammary carcinomas, ALDH1+ CD44- Ki67+ CSCs (CSC2s) constitute about 5.54% of the tumor mass and lie in a sub-central site, physically distinct from ALDH1+ CD44+ Ki67+ CSCs (CSC1s) (Liu et al., 2014).