Considering that ABCG2 upregulation is influenced by HIF-1α activation (39) and that the level of HIF-1α expression is frequently overexpressed in tumors, such as anaplastic thyroid cancer, with a strong association to the aggressive disease phenotype and the therapeutic resistance (40), we have decided to evaluate also the effects of topotecan and pazopanib combination on HIF-1α expression. This evidence concerns the gene ABCG2 and thyroid gland undifferentiated (anaplastic) carcinoma.