Glycoprotein (GP) IX is a platelet membrane 20 kD protein, which is associated with GPIbα, GPIbβ, and GPV to form GPIb/IX/V complex. GPIb/IX/V complex is a major receptor for von Willebrand factor, which mediates platelet adhesion and aggregation under high shear stress conditions. The relevance of this receptor for hemostasis has been implicated by a congenital bleeding disorder lacking the receptor, Bernard-Soulier syndrome. All subunits for the human receptor have been cloned and characterized. However, the function of GPIX is still elusive. To obtain further information of GPIX, we have determined a cDNA sequence of mouse GPIX (811 bp). The deduced amino-acid sequence (177aa) was 71% identical to the human GPIX protein. All cysteine residues in extracytoplasmic domain and putative N-linked glycosylation site (Asn44) were conserved. Mouse GPIX contained a leucine-rich glycoprotein sequence composed of 24 amino acids, as did human GPIX. Blood Coag Fibrinol 9:381–385 × 1998 Lippincott-Raven Publishers.