Earlier studies had shown that the expression of the gene coding for one eminent connective tissue proteoglycan, decorin (DCN), is deficient in the fibroblasts of 3 out of 15 Marfan patients (Pulkkinen et al., 1990). To obtain more information on the expression of this gene, various human tissues and cell lines were studied. High mRNA levels of decorin were detected in aorta, lung, skin, kidney, smooth muscle, and placenta, whereas significantly lower mRNA levels were found in the rest of the tissues analyzed. Two sizes of transcripts were observed in all tissues. The two transcripts of decorin most probably do not represent two different genes, since in situ hybridization gave only one strong signal, placing the gene in 12q21----q22. No tissue-specific differences in the two mRNA species of decorin were detected. This is in contrast to the gene of versican, another connective tissue proteoglycan gene, that was analyzed as a control; high expression of a longer transcript of the versican gene was found in brain and smooth muscle, whereas the shorter transcript was predominant in all other tissues studied. DCN was actively transcribed in cultured mesenchymal cells, whereas in cells of endothelial or epithelial origin, the transcription level was undetectable. These tissue- and cell type-specific variations in the expression of DCN may help to explain the complex phenotypic variation typical of individuals with Marfan syndrome.