ABCG2 and head and neck squamous cell carcinoma: mRNA expression levels of a subset of drug transporters (ABCB1, ABCC1, ABCC2, ABCG2, SLC31A1) were determined by qRT-PCR in a study sample of therapy naïve stage IVa HNSCC tumors (n = 40) and normal control samples (n = 14) (Fig. 1A).