Bone metabolism related protein and gene thereof

A method of assaying a function or an activity of an osteoblast differentiation promoting factor (OBDPF) protein, said method comprising:

• culturing a cell expressing an OBDPF protein, and
• detecting a function or activity of said OBDPF protein in said cell;
• wherein said cell is an osteoblast, and said function or activity is promoting differentiation of the osteoblast to change from an immature state to a mature state;
• said OBDPF protein is a polypeptide comprising the amino acid sequence of SEQ ID NO:2 or SEQ ID NO:4, or a polypeptide encoded by a nucleic acid which hybridizes under stringent conditions with a nucleic acid comprising the nucleotide sequence of SEQ ID NO:1 or SEQ ID NO:3,
• said hybridization being carried out by conducting hybridization for 16 hours in 6×SSC or a hybridization solution having an equivalent salt concentration at a temperature of 50 to 60° C., followed by preliminary washing with 6×SSC or a solution having an equivalent salt concentration as necessary, and subsequently washing with 1×SSC or a solution having an equivalent salt concentration.