A method for selecting or designing a compound expected to modulate the activity of Leukotriene C4 synthase (LTC4S), the method comprising the step of using molecular modelling means to select or design a compound that is predicted to interact with the catalytic site or a substrate binding region of LTC4S, wherein a three-dimensional structure of at least a part of the catalytic site or a substrate binding region of LTC4S is compared with a three-dimensional structure of a compound, and a compound that is predicted to interact with the said catalytic site or substrate binding region is selected. The selected compound may be predicted to bind to at least a part of a region of the structure termed the “GSH substrate binding cavity” (formed by residues including residues Arg51, Arg30, Arg104, Gln53, Asn55, Glu58, Tyr59, Tyr93, Tyr97, Ile27, Pro37, Leu108 of full length human LTC4S, or equivalent residues); the “lipophilic substrate binding crevice” (formed by residues including Ala20, Leu24, Ile27, Tyr59, Trp116, Ala112, Leu115, Leu108, Tyr109, Leu62, Val119, Thr66, Vall6 and Leu17, or equivalent residues); or the “catalytic site” (formed by residues including Arg104 or Arg31, or equivalent residues).