Abstract The major pathogenic species of Apicomplexa are all intracellular eukaryoticparasites that generally reproduce within a parasitophorous vacuole (PV) in infected hostcells. This niche presents both an opportunity and a challenge to the parasites growingwithin ? on the one hand, they are sequestered from some of the immune defenses thatmight detect their presence but at the same time, the PV membrane (PVM) presents aphysical barrier to the export of protein effectors necessary to modulate host functions tothe parasite's advantage. The phylum Apicomplexa includes many important human and animal pathogensincluding Plasmodium sp., the cause of human malaria, Cryptosporidium parvum, thecause of debilitating diarrheal disease, and Toxoplasma gondii, the cause of seriousneurologic disease in the developing fetus and those who are immunocompromisedthrough cancer (e.g., lymphoma), transplantation or infections, such as HIV-AIDS. Wehave recently determined that Toxoplasma tachyzoites have the ability to dramaticallyand specifically up-regulate expression of the human oncogene, c-Myc, using an effectorreleased from dense granules, called GRA16. Using a genetic screen for mutantsdefective in c-Myc up-regulation, we have also recently identified the first components ofthe Toxoplasma machinery that translocates GRA16 and other dense granule effectorsacross the PVM. These novel proteins have been dubbed MYR1, MYR2 and MYR3. Thegoal of the work proposed here is to identify the complete or near completemachinery involved in translocation of effectors across the PVM, determine thehost responses that are dependent on this machinery, and elucidate theimportance of these effectors in Toxoplasma pathogenesis. We will do this throughusing a combination of biochemical and genetic approaches. Through this work, we will both unveil an important and novel piece of cellbiology and identify crucial parasite components for eventual chemotherapeutic targetingand amelioration of the disease caused by this and related parasites.