Site-directed antibodies’ transmembrane topology mapping of untagged NaPi2b in live and intact ovarian cancer cells allowed us to experimentally validate a previously predicted model for the transmembrane organization of extramembrane domains (Yin et al., 2008) and provide an up-to date experimental platform for NaPi2b epitope-based cancer immunodiagnostics and immunotherapy. The gene discussed is SLC34A2; the disease is ovarian cancer.