ALDH1A1 and thyroid cancer: As CD44s has a property as a marker of tumor-initiating cells in several solid malignancies [22], [26]–[29], we tested whether or not there is an association between the expression of CD44s and human aldehyde dehydrogenase 1 (ALDH1) enzymatic activity, which is another marker of tumor-initiating cells [39], [40], in thyroid cancer cell lines.