The neu gene in rat neuro/glioblastoma was found to be activated by a single point mutation in the DNA sequence encoding the transmembrane region of the neu-encoded p185 protein. The human homologue of the rat neu gene, termed c-erbB-2 or HER-2, can also be activated in vitro by a similar mutation in the corresponding region. Although the human neu gene was shown to be amplified/overexpressed in a large portion of human breast and ovarian cancer, no reports indicate that the human neu gene is activated by a point mutation in human tumor. To study the possible point mutation of neu gene in human tumors, we characterized the genomic structure in the transmembrane region of human neu gene, which in turn allowed us to determine DNA sequence in this region directly following DNA amplification by polymerase chain reaction. We analyzed 7 tumor cell lines (2 breast cancer, 1 neuroblastoma, 1 rhabdomyosarcoma, and 3 glioma) and 11 tumor tissue samples (8 breast and 3 ovarian cancers). No mutation was found in the transmembrane region of human neu gene. Our results suggest that unlike the rat neuro/glioblastoma, the single point mutation in the transmembrane region of the human neu gene is a rare event in human tumors. In this study, we developed a technique for direct DNA sequencing of the transmembrane region of the human neu gene. This technique makes it possible to screen a large number of tumor samples.