TPBG is a 420 amino acid single-pass type I membrane protein expressed by all types of trophoblasts as early as 9 weeks of development. TPBG contains an N-terminal putative signal sequence, a 310-residue extracellular region, a membrane anchorage domain and a 44-amino acid cytoplasmic tail with a potential phosphorylation site. The extracellular region has seven potential N-glycosylation sites and seven leucine-rich repeats, which are located in two regions separated by a hydrophilic stretch.