Cloning of CCK-B/gastrin receptor cDNAs showed that they possess the feature of serpentine G protein coupled receptors. In this study, the receptor mRNA was detected selectively in all SCLCs (10 cases) with a RT-PCR assay. By contrast, it was detectable in only one of thirteen squamous cell carcinomas or twenty-one adenocarcinomas of lung. Thus, CCK-B/gastrin receptor has been demonstrated to provide a novel molecular marker for the diagnosis of small cell lung cancer (SCLC) by using biopsy specimens.