The main finding of our study was that brain delivery of [11C]tariquidar was comparably low in tumor and tumor-free brain tissue, which suggested that ABCB1/ABCG2 transport activity was sufficiently intact in tumor tissue to restrict brain entry of anticancer drugs which are dual ABCB1/ABCG2 substrates. The gene discussed is ABCG2; the disease is neoplasm.