Biglycan (PG-I, DS-PG-1, PG-S1) is a small cellular or pericellular matrix proteoglycan that is closely related in structure to two other small proteoglycans, decorin (PG-II, PG-S2, DS-PG2, or PG-40) and fibromodulin. The core protein is made up predominantly of a series of 11 tandem repeats that appear to have been used throughout evolution for protein-protein, protein-cell, or cell-cell interactions. The function of biglycan is unclear at this time, but it has been shown to bind transforming growth factor beta in vitro. We have cloned and partially sequenced the approximately 8-kilobase pair human biglycan gene. The gene consists of eight exons including one in the sequence that encodes the 5'-untranslated region of the mRNA. The first and seventh introns are approximately 1 kilobase pair, while the remainder are shorter. With the exception of the first two introns, all of the introns are spread throughout the hydrophobic repeat domain. The 500-base pair 5' to the start of transcription contains several elements that strongly suggest that it contains a significant amount of the gene promoter. The elements include one AP2 and five SP1 consensus sequences. Like in many other genes, the biglycan gene promoter lacks both a CAAT and TATA box but is rich in GC content. Using 3H-labeled cDNA and in situ hybridization and autoradiography of human chromosomes, the human gene was localized to the end of the long arm of the X chromosome (Xq27-ter). The relationship of biglycan to a number of other proteins containing the leucine-rich repeats is discussed with respect to homologies of cysteine regions immediately adjacent to the repeat sequences.