Three alleles of the Deficiens-homologous potato gene St-Deficiens (St-Def) present in the genome of a tetraploid Solanum tuberosum variety were isolated and characterized. For one allele (St-Def pD13) the complete molecular structure was determined by sequence analysis and comparison with its cDNA, while for the other two alleles (pD10, pD12) only partial sequences of regulatory and coding regions were obtained. All three alleles showed (except for one amino acid exchange in pD10) identical sequences in the coding region. While sequence variation was observed within the respective promoters starting some 300 nt upstream of the putative transcriptional start site, the 3' terminal promoter sequences were highly conserved. Within this region, a sequence of 81 nt was identified which showed 73% sequence identity to a corresponding sequence in the Deficiens promoter. This region which contains a putative serum response element was previously shown to regulate the expression of the Deficiens gene in Antirrhinum majus. Expression patterns for the three alleles in transgenic potato lines expressing St-Def promoter/Gus constructs were identical. GUS activity was predominantly located in petals and stamens as expected for the activity of a Def-homologous gene, but a significant level of expression was also detected in the ovary wall and in the vascular bundles supporting anthers and petals. The promoters were also active in abscission structures at the junction of flower stem and pedicel, as well as in anther stomium.