Apical plasma membrane localization of prostate specific membrane antigen (PSMA) in situ. PSMA is an important clinical biomarker and a potential antigenic target for immunotherapy of prostate cancer. Immunofluorescence analysis and confocal microscopy reveal glandular architecture of the prostatic epithelium, with propidium iodide-stained nuclei in red. PSMA (green) is predominantly localized to the apical plasma membrane surface of the prostatic epithelium, lining the lumenal surface of the glands. This pattern of subcellular localization may have important potential consequences related to the accessibility of PSMA to intravenously administered monoclonal antibodies. For details, see Christiansen et al. in this issue.