Plasmodium falciparum malaria is treated using artemisinin combination therapy (ACT), in which artemisinin is supplied along with partner drugs such as mefloquine, piperaquine, and lumefantrine. However, resistance has been reported in endemic regions. To identifying new effector genes involved in resistance, Iwanaga et al. develop a large scale transgenic screen with genomic libraries of resistance strains. Using this approach they provide evidence that transcriptional upregulation of pfmdr7 contributes to mefloquine resistance in a clinical isolate.