Although involved in the modulation of tumor aggressiveness, both CD133 (74, 217, 219) and CD44 (67, 243) do not appear to be essential for cancer formation, leading to the inference that these markers represent markers of more differentiated progenitor cells in the hierarchy although they may be co-expressed with nestin and GFAP in “higher up” progenitors. Here, CD44 is linked to cancer.