The genes for SHBG/ABP and the SHBG-like region of vitamin K-dependent protein S have evolved from a common ancestral gene

Sex hormone binding globulin (SHBG) is the most important sex steroid transport protein in human plasma. It is the product of the same single gene as the androgen binding protein (ABP) of testis. Protein S is another protein, which is an important cofactor in the anticoagulation system and, as far as is known today, functionally unrelated to SHBG/ABP. Protein S also has a role in the complement system. A comparison of the human genes for SHBG/ABP and protein S reveals a sequence similarity, which is of a low grade only, between the SHBG/ABP protein and a similar sized COOH-terminal domain of protein S. However, the intron-exon organization exhibits a striking similarity in the two genes, illustrating evolutionary events leading to the appearance of two functionally different proteins from common ancestral genetic elements.