In comparison with animals, relatively few plant genes have been identified that have been shown to be under organ-, tissue- or cell-type-specific regulation. In this paper, we describe how the beta-glucuronidase (GUS) reporter gene (gusA or uidA), fused to a weak promoter (a truncated (-90 bp) CaMV35S promoter), can be used to identify tissue-specific markers in transgenic tobacco plants. The rationale was that the expression of gusA would be determined primarily by position effect. Quantitative analysis revealed that, of 184 -90-gus transgenic plants, 73% exhibited gusA gene activation in leaf tissue, and the level of GUS enzyme activity varied over a 300-fold range within the population. In comparison, transformation with a promoterless gusA gene resulted in GUS expression in 78% of all plants analyzed (in leaf and/or root) and expression levels were three-fold or more lower. Qualitative GUS analysis of single locus -90-gus transformants revealed differential expression in diverse tissues. The spatial pattern of GUS activity was unique to individual transformants, was a reflection of differential gusA gene transcription, and was stably transmissible to progeny. Evidence for preferential expression in roots not only of the -90-gus, but also the promoterless gusA gene is presented. The value of the -90 bp promoter-gusA sequence, which is termed an 'interposon', as a tool both to identify native enhancer sequences in situ and to investigate position effects in plants, is discussed.