The level of CD133 expression in circulating tumor cells of patients with thyroid cancer has previously been shown to vary by level of differentiation of the tumor, with higher levels noted in poorly differentiated and anaplastic thyroid cancer.20 Others have found by IHC that CD133 is expressed in 61% of PTC tumors and not associated with any clinical features thought to be indicative of high recurrence risk, suggesting it may be a biologic marker that could be additive to clinical risk factors, similar to our findings in the current study.21 Here, PROM1 is linked to thyroid gland undifferentiated (anaplastic) carcinoma.