Monoamine oxidase from Aspergillus niger (MAO-N) is an FAD-dependent enzyme that catalyses the conversion of terminal amines to their corresponding aldehydes. Variants of MAO-N produced by directed evolution have been shown to possess altered substrate specificity. Crystals of two of these variants (MAO-N-3 and MAO-N-5) have been obtained; the former displays P21 symmetry with eight molecules per asymmetric unit and the latter has P4 1212 or P43212 symmetry and two molecules per asymmetric unit. Solution of these structures will help shed light on the molecular determinants of improved activity and high enantioselectivity towards a broad range of substrates. © 2008 International Union of Crystallography All rights reserved.