a, Phenotype of P. falciparum in peripheral blood of NOD-scid IL-2R_null mice engrafted with human erythrocytes. Blood samples were taken at days 5 and 7 of the assay (one and two asexual cycles, respectively) after the start of treatment with vehicle or DDD107498 at day 3. The bi-dimensional flow cytometry plots measure the murine (Ter-119-PE+) and human (Ter-119-PE−) erythrocytes, and the presence of nucleic acids (infected SYTO-16+ events). The blue circles indicate the region of infected erythrocytes. Vehicle-treated mice showed a characteristic pattern of staining with SYTO-16 (ref. 35), which correlated with the presence of healthy rings, trophozoites and schizonts in blood smears. Conversely, mice treated with DDD107498 at 50 mg per kg showed only trophozoites with condensed cytoplasm and some pyknotic cells at day 5 (red circle in flow cytometry plot and corresponding blood smears). By day 7, few infected erythrocytes were detected by flow cytometry and blood smears revealed parasites with a similar morphology to those at day 5. This suggests that trophozoites are the most sensitive population since the cycle is interrupted at this stage. The images displayed are taken from a mouse with high levels of parasitaemia. At least 50 parasites were counted per sample screened in the microscope. Of these, four photographs of representative parasite phenotypes were selected to represent the morphology of the most prevalent phenotype. Thus, this is a qualitative assessment. b, Stage specificity assays using synchronized cultures. For morphological analysis of antimalarial drug action, thin blood smears were prepared, fixed and stained with Giemsa followed by examination with an upright microscope using an oil-immersion lens (×100). For parasitaemia determination, a total number of 1,000 red blood cells (corresponding to five microscopic fields) were counted. R to T, abnormal trophozoites observed after exposure for 24 h of synchronized rings to DDD107498. T to S, trophozoites do not develop into schizonts after exposure for 24 h to DDD107498. S to R, no ring stages are observed 24 h after treatment of schizonts with DDD107498. c, Percentage parasitaemia in the red blood cells. R, ring stage; T, trophozoite; S, schizont.