Homospermidine synthase. which catalyses the first pathway-specific reaction in pyrrolizidine alkaloid biosynthesis, was cloned from root cultures of Senecio vulgaris and expressed in E. coli. The open reading frame encodes a protein of 370 amino acids with a molecular mass of 40,740 Da. The enzyme is strictly dependent on spermidine as aminobutyl donor since it cannot be substituted by putrescine. The homospermidine synthase from S. vulgaris showed 97.9 and 99.3% nucleic acid identity with two HSS sequences from the closely related species Senecio vernalis. This report also revises data from a previous publication (Kaiser, A., 1999. Cloning and expression of a cDNA encoding homospermidine synthase from Senecio vulgaris (Asteraceae) in Escherichia coli. Plant J. 19. 195 201.) that is incorrect.