The insulin-like growth factor (IGF) binding protein superfamily consists of six high-affinity IGF binding proteins (IGFBPs) and at least four low-affinity binding proteins known as IGFBP-related proteins (IGFBP-rPs). All members of the IGFBP superfamily are cysteine-rich proteins with conserved cysteine residues.IGFBPs contribute to the prolongation of the half-life of IGF in all tissues, including the prostate. In humans, insulin-like growth factor binding protein 6 (IGFBP-6) is a protein encoded by the IGFBP6 gene.IGFBP-6 binds IGF-II with higher affinity than IGF-I and it is an inhibitor of IGF-II specifically. IGFBP-6 is reported to be IGF-independent and plays a role in cell proliferation, survival, differentiation, and migration.IGFBP-6 is unique among IGFBPs due to its apparent IGF-II binding preference.