Notably, the IC50 values of the Plrx parasite line did not differ significantly from WT parasites, excluding, at least ex vivo, a central function of Plrx in anti-redox stress defense. To elucidate whether this dispensability can be observed in vivo we applied a growth assay as outlined above under enhanced oxidative stress conditions. We selected methylene blue as this antimalarial was shown to challenge the parasites intracellular reducing milieu through the generation of pro-oxidant H2O2. Since our in vitro and in vivo data exclude an essential function of Plrx to maintain the parasite��s redox equilibrium, we extended our analysis of the Plrx-deficient strain to expression profiling of selected redox proteins. This analysis was expected to further reveal the modulation of intracellular redox networks. We studied the effects of the Plrx-deletion on mRNA levels of genes related to cellular redox metabolism with a focus on the cytosolic components because Plrx is a cytosolic member of the antioxidant network. Gene transcript levels were measured by quantitative real-time RT-PCR and the effect on a target gene is reported as differences in comparison to a WT control population. Transcript levels of mRNA for the two major sustainers of redox homeostasis, thioredoxin reductase and glutathione reductase increased only slightly. Plrx was previously shown to directly interact with these two systems, thereby potentially acting as an additional antioxidant defence line of Plasmodium. The prediction was that deletion of Plrx may be accompanied by a compensatory upregulation of functional paralogues that balance the reducing capacity of Plrx. Such a function can most likely be fulfilled by thioredoxin and/or glutathione. While this assumption is supported by the weak increase of Trx mRNA levels, GSH cannot be tested directly because it is only a tripeptide. Another member of the thioredoxin superfamily, glutaredoxin did not change significantly. Moreover, Plrx-deficient parasites show a slight decrease in mRNA levels of thioredoxin peroxidase 1, the major cytosolic peroxiredoxin of the parasite, and ribonucleotide reductase. Collectively, these data show that depletion of PbPlrx caused only weak alterations in gene expression of selected members of the cytosolic redox FDA-approved Compound Library network compared to wild type parasites. Since we could exclude a discernible function of Plrx in blood stage development we extended our phenotypic analysis to the entire Plasmodium life cycle. Plrx parasites did not differ from WT parasites in sexual development, which is a prerequisite for transmission to mosquitoes. Dissection of infected mosquitoes showed similar numbers of oocysts, midgutassociated and salivary gland-associated sporozoites in WT and Plrx deficient parasites. Therefore, Plrx is also dispensable for sporogony and sporozoite maturation. When mature salivary gland sporozoites were tested for infectivity to the mammalian host in vivo and in vitro again no phenotypic differences between the two parasite lines could be observed. Hepatocytes infected with Plrx sporozoites were indistinguishable from WT infected cells and produced high numbers of mature liver stage parasites. When tested in vivo by intravenous injection or natural mosquito bite the recipient animals became patent after similar prepatent periods compared to WT sporozoite inoculation. Together these data exclude a vital role for Plrx in Plasmodium life cycle progression under standard conditions. We initiated this study to test the potential of plasmoredoxin, a Plasmodium specific member of the thioredoxin superfamily, as a novel antimalarial drug target. Using classical reverse genetics we could demonstrate that Plrx is dispensable for Plasmodium development OSI-774 inside its host cells. This finding rejects future drug discovery efforts that aim at specifically targeting Plrx, most likely even in combination with existing antimalarial drugs. Successful generation of Plrx mutants permitted a detailed observation of the in vivo function of Plrx during life cycle progression of the malaria parasite. Again, no vital role at any stage of the parasite life cycle was revealed. Therefore, specific targeting of plasmoredoxin is not suitable either for transmissionblocking or causal-prophylactic malaria intervention strategies. The redox-active proteins thioredoxin and glutaredoxin are founding members of the thioredoxin superfamily. Additional members include tryparedoxin of Trypanosomes, the protein disulfide isomerase and a few bacterial disulfide bondforming proteins.