Human plasma sex hormone-binding globulin (SHBG) and testicular androgen-binding protein (ABP) are homodimeric glycoproteins with a single steroid-binding site. They share the same primary structure and differ only with respect to the types of oligosaccharides attached to them. Both are products of a single gene ( Shbg), the expression of which has been detected in several tissues including liver, testis, placenta, brain and endometrium. The transcript encoding SHBG in hepatocytes and ABP in Sertoli cells is identical. Several other transcripts result from differential exon utilization in sex steroid-responsive tissues, but their biological significance remains obscure. Wild-type and mutant forms of human SHBG have been produced in mammalian cells and Escherichia coli, and have provided insight into the structural and functional properties of SHBG and its related gene products.