ABCB4 and medullary thyroid gland carcinoma: Chemotherapy failure has been ascribed, at least in part, to the overexpression by MTC cells of the multidrug resistance proteins (MDRs), especially MDR1 and MDR2, transmembrane glycoproteins that antagonize intracellular accumulation of cytotoxic agents (Yang et al. 1991; Massart et al. 1995; Massart et al. 1996a; Massart et al. 1996b; Zatelli et al. 2005; Ruggeri et al. 2006).