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Giardia lamblia (GL) is a flagellated protozoan that resides in the intestines of different vertebrate hosts. GL is the most frequent cause of waterborne diarrhea worldwide (1). The disease can cause severe malnutrition (2), and infection has been correlated with impaired cognitive development in less-developed countries (3). GL, considered one of the most ancient eukaryotes (4-6), is a diplomonad with a simple life cycle. Infection occurs after the ingestion of cysts contaminating water or food or through person-to-person transmission. Excystation in the duodenum results in the development and release of trophozoites that proliferate in the small intestine. Cysts develop as the trophozoites are swept down the intestine. Importantly, in vitro cholesterol starvation induces trophozoites to differentiate into environmentally resistant cysts (7), a process that occurs in the lower small intestine after the absorption of bile salts and cholesterol. The precise mechanism of cyst wall formation is unknown, but its assembly is preceded by the synthesis, packing, and release of secretory components destined for the cyst wall, such as cyst-wall proteins 1 (CWP-1) and 2 (CWP-2). Upon formation, the cysts are passed in the feces, and the life cycle is completed.The sterol regulatory element binding-proteins (SREBPs) were discovered in mammalian cells (8). SREBPs are pivotal transcription factors of the genes of lipid metabolism. Cholesterol starvation activates a proteolytic cascade that Abbreviations: CHO, Chinese hamster ovary; CWP, cyst wall protein; GL, Giardia lamblia ; mSREBP, mature SREBP; RLU, relative light units; SRE, sterol regulatory element; SREBP, sterol regulatory element binding protein.