Midkine (MK) and its functionally-related protein Pleiotrophin are heparin-binding neurotrophic factors that signal through the same receptor, known as anaplastic lymphoma kinase (ALK). MK plays an important regulatory role in epithelial-mesenchymal interactions during fetal development and in postnatal lung development. MK chemoattracts embryonic neurons, neutrophils and macrophages, and by signaling through the ALK receptor it exerts angiogenic, growth and survival activities during tumorgenesis. Recombinant human Midkine is a 13.4 kDa protein containing 123 amino acid residues including five intra-molecular disulfide bonds.