Each of the two TMDs comprise six transmembranealpha helices, and the two NBDs have binding sites for ATP that are homologousthroughout the superfamily, unlike the TMDs.19 As its name suggests, MRP4 can confer resistance to drugs, including cancerchemotherapy, antivirals, and antibiotics.20 It can also transport signaling molecules such as cyclic nucleotides andeicosanoids, making it a drug target for inflammation, pain,21 and cardiovascular disease,22 and it has also been implicated in the development of cancer.23,24 To date, thereis no known structure of human MRP4. This evidence concerns the gene ABCC4 and cancer.