Reddy et al. (1998) cloned human OSTF1, which they designated OSF, from an osteoclast-like multinucleated cell (OST-like MNC) expression library. The deduced 214-amino acid OSTF1 protein has a predicted molecular mass of 28 kD and shares 95% sequence identity with the mouse homolog. It contains an N-terminal proline-rich region and a potential N-glycosylation site, but lacks a signal sequence. OSTF1 has ankyrin repeats and an Src homology-3 (SH3) domain that is homologous to the SH3 domain of GRB2 and FYN. Northern blot analysis detected ubiquitous expression of a 1.3-kb transcript in multiple human tissues. In situ hybridization demonstrated OSTF1 expression in giant cells isolated from osteosarcoma and in osteoclast-like cells in long-term bone marrow cultures.