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Entamoeba histolytica, the causative agent of human amoebiasis, is essentially anaerobic, requiring a small amount of oxygen for growth. It cannot tolerate the higher concentration of oxygen present in human tissues or blood. However, during tissue invasion it is exposed to a higher level of oxygen, leading to oxygen stress. Cysteine, which is a vital thiol in E. histolytica, plays an essential role in its oxygen-defence mechanisms. The major route of cysteine biosynthesis in this parasite is the condensation of O-acetylserine with sulfide by the de novo cysteine-biosynthetic pathway, which involves cysteine synthase (EhCS) as a key enzyme. In this study, EhCS was cloned, expressed in Escherichia coli and purified by affinity and size-exclusion chromatography. The purified protein was crystallized in space group P41 with two molecules per asymmetric unit and a complete data set was collected to a resolution of 1.86 …