Insulin like growth factor binding protein 3 (IGFBP3) is a member of the superfamily of insulin like growth factor (IGF) binding proteins which include six high affinity IGF binding proteins (IGFBP) and at least four low affinity binding proteins referred to as IGFBP related proteins (IGFBPrP). The IGFBP members are cysteine rich proteins with conserved cysteine residues clustered in the amino terminal and the carboxy terminal regions of the molecule. The cDNA sequence encoding the mature human IGFBP3 is fused to the signal peptide of CD33.