Comparative analysis of proteoglycans in the control and tumor tissue of human mammary gland revealed disorders in the biosynthesis of dermatan sulfate proteoglycans in tumor cells. Using the methods of reverse transcription-PCR and Western blotting for the analysis of decorin expression we showed that these disorders were paralleled by reduced expression of the core protein and changes in the structure of proteoglycan carbohydrate chains and could be a cause of malignant degeneration of mammary gland cells.