GDF-9 is a member of the TGF-β superfamily of growth and differentiation factors, and is highly homologous to GDF-3. Unlike most TGF-family members, GDF-9 and GDF-3 are not disulfide-linked dimers. GDF-3 is expressed in adult bone marrow, spleen, thymus, and adipose tissue. GDF-9 is expressed in oocytes and is required for normal ovarian folliculogenesis. Human GDF-9 is a 31.0 kDa non-disulfide-linked homodimer containing two 135 amino acid polypeptide chains.