STIM1 is a novel candidate growth suppressor gene mapping to the human chromosome region 11p15.5 that is associated with several malignancies. STIM1 overexpression studies in G401 rhabdoid tumour, rhabdomyosarcoma and rodent myoblast cell lines causes growth arrest, consistent with a potential role as a tumour growth suppressor. We used highly specific antibodies to show by immunofluorescence and cell surface biotinylation studies that STIM1 is located at the cell surface of K562 cells. Western blot analysis revealed that the 90-kDa STIM1 protein is ubiquitously expressed in various human primary cells and tumour cell lines. STIM1 is not secreted from cells and does not appear to undergo proteolytic processing. We show evidence of post-translational modification of STIM1, namely phosphorylation and N-linked glycosylation. Phosphorylation of STIM1 in vivo occurs predominantly on serine residues. Thus, STIM1, the putative tumour growth suppressor gene is ubiquitously expressed and has features of a regulatory cell-surface phosphoprotein.