CD44 and neoplasm: CSCs derived from the primary tumor mass (primary CSC) generate transit-amplifying progenitors and their short-lived derivatives (i.e., clones of bulk cells) with phenotypic and functional heterogeneity, but without tumor-initiating capacity.15,23 Individual CD44+/CD24–/low stem-like cells are detectable in the tumor-invasive edge adjacent to the tumor stroma (Fig. 1a).