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Acute intermittent porphyria (AIP) is a disorder of the haem biosynthetic pathway caused by mutations in the hydroxymethylbilane synthase (HMBS) gene. Knowledge of the spectrum of mutations present in South Africa is limited. This study presents the molecular profile of 20 South African patients with AIP, and the kinetic analysis of one novel expressed mutated HMBS enzyme and a previously identified mutation at the same position.Genomic DNA was isolated from affected probands and selected family members, the HMBS gene amplified and mutations characterised by direct sequencing and restriction enzyme analysis. One of the novel mutations (p.Lys98Glu), a previously characterised mutation at the same position (p.Lys98Arg), and the wild-type enzyme were expressed, purified and subjected to partial kinetic characterisation.Four new mutations, p.Lys98Glu, p.Asp230Aspfs*20, c.161-1G>A and c.422+3_6delAAGT, are described. Seven previously described mutations were found, while four patients revealed no mutations. Mutation analysis of five offspring of one of the probands carrying the p.Trp283X mutation revealed two asymptomatic carriers. Kinetic analysis showed that the p.Lys98Glu mutation results in loss of substrate affinity, whereas the previously described p.Lys98Arg mutation causes the loss of binding between the enzyme and its dipyrromethane cofactor, rendering the enzyme inactive.This study comprises the most comprehensive characterisation of HMBS gene mutations in patients with AIP in South Africa. The biochemical characterisation of expressed HMBS mutants reveals insight into the mechanism of catalytic activity loss, which may inspire investigation into individualised therapy based on the molecular lesion identified.