CD44 and cancer: In a wide variety of solid tumors, including those of breast [6], gastric [26], pancreas [27], ovary and colon [28,29,30], and also in blood malignancies [31], residence in, or transition into the aggressive tumor-initiating cell state can be monitored by high expression of the cancer stem cell marker CD44—where the nomenclature CD44Hi represents cells enriched for that aggressive cancer cell phenotype [32,33,34,35,36].