 It has a large extracellular domain of about 561 amino acids, a hydrophobic transmembrane domain and a short cytoplasmic tail domain composed of 45 amino acids.The 260 amino acid region closest to the extracellular membrane is referred to as the ZP domain (or, more correctly, ZP module).The outermost extracellular region is termed as the orphan domain and it is the part that binds ligands such as BMP-9.