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Abstract: Human leukotriene C4 synthase (LTC4S) forms highly ordered two-dimensional (2D) crystals under specific reconstitution conditions. It was found that control of a larger number of parameters than is usually observed for 2D crystallization of membrane proteins was necessary to induce crystal formation of LTC4S. Here, we describe the parameters that were optimized to yield large and well-ordered 2D crystals of LTC4S. Careful fractioning of eluates during the protein purification was essential for obtaining crystals. While the lipid-to-protein ratio was critical in obtaining order, four parameters were decisive in inducing growth of crystals that were up to several microns in size. To obtain a favorable diameter, salt, temperature, glycerol, and initial detergent concentration had to be controlled with great care. Interestingly, several crystal forms could be grown, namely the plane group symmetries of p2, p3, p312, and two different unit cell sizes of plane group symmetry p321. [Copyright &y& Elsevier]