CD44 and cancer: Kryczek et al. have explored the expression of various putative stem cell markers, including CD133, CD117, CD44, ABCG2, ESA (Epithelial Specific Antigen) and ALDH, in primary ovarian cancer samples and observed that: (i) these markers exhibited a highly heterogeneous expression; (ii) only CD133 and ALDH enriched for cancer-initiating cells, as assayed both in vitro by tumor sphere assay and in vivo after inoculation in NOD/SCID mice [205].