Patatin is a family of glycoproteins that contributes about 40% of the total soluble protein in tubers of potato (Solanum tuberosum L.). The protein is encoded by a multigene family of 50-70 genes which have been divided into classes I and II on the basis of sequence homology. The promoters of two class I genes, PS20 and PS3/27, were transcriptionally fused to beta-glucuronidase and transformed into the potato cultivars Désirée and Maris Bard. Examination of the expression levels in large populations of microtubers indicated that the PS20 promoter produced beta-glucuronidase activities 5-fold lower in Désirée than Maris Bard whereas the PS3/27 promoter showed similar levels in both cultivars. Furthermore, the relative expression levels from the two promoters were reversed in the two cultivars. The beta-glucuronidase enzyme activity was correlated with the mRNA level but not the copy number of the introduced gene. The implications for the use of patatin promoters in the genetic modification of tubers is discussed.