Biochemical and physiological features shared among MDR tumor cells, malarial parasites and certain Gram-negative bacteria include altered intracellular accumulation of the drugs to which cells or micro-organisms are resistant, and that certain 'chemo-modulators' (e.g., the calcium channel blocker, verapamil [VPL]) can partially reverse the drug resistance. Although it is still not precisely understood how VPL functions as a chemoreversal agent, it is reasonable to expect that concepts from the study of MDR tumor cells will apply to MDR malarial parasites, and vice versa. Such was the hope when pfmdr genes were first cloned [38, 39] and shown to be homologous to the hsmdr1 gene, which is overexpressed in MDR tumor cells. It was initially thought that an ABCprotein drug pump for CQ must therefore exist in drug-resistant P. falciparum, similar to the drug pump proposed for tumor cells (human multidrug resistance protein 1 [HsMDR1], also known as Pgp) believed by many investigators to directly translocate vinblastine and many other anti-tumor drugs.