Microarray and real-time RT-PCR were used to examine expression changes in primary bone marrow cells and RAW 264.7 cells in response to RANKL. In silico sequence analysis was performed on a novel gene which we designate OC-STAMP. Specific siRNA and antibodies were used to inhibit OC-STAMP RNA and protein, respectively, and tartrate-resistant acid phosphatase (TRAP)+ multinucleated osteoclasts were counted. Antibodies were used to probe bone tissues and western blots of RAW cell extracts +/- RANKL. cDNA overexpression constructs were transfected into RAW cells and the effect on RANKL-induced differentiation was studied. OC-STAMP was very strongly up-regulated during osteoclast differentiation. Northern blots and sequence analysis revealed two transcripts of 2 and 3.7 kb differing only in 3'UTR length, consistent with predictions from genome sequence. The mRNA encodes a 498 amino acid, multipass transmembrane protein that is highly conserved in mammals. It has little overall homology to other proteins. The carboxy-terminal 193 amino acids, however, are significantly similar to the DC-STAMP family consensus sequence. DC-STAMP is a transmembrane protein required for osteoclast precursor fusion. Knockdown of OC-STAMP mRNA by siRNA and protein inhibition by antibodies significantly suppressed the formation of TRAP+, multinucleated cells in differentiating osteoclast cultures, with many TRAP+ mononuclear cells present. Conversely, overexpression of OC-STAMP increased osteoclastic differentiation of RAW 264.7 cells. We conclude that OC-STAMP is a previously unknown, RANKL-induced, multipass transmembrane protein that promotes the formation of multinucleated osteoclasts.