The expression product of ct120a, a novel gene isolated from human chromosome 17p13.3 in our laboratory, was predicted to have seven transmembrane domains and could cause malignant transformation of mouse NIH3T3 cells. There existed an mRNA splicing variant of ct120a, namely ct120b, which had a 96-nucleotide deletion and produced an in-frame loss of 32 amino acids from codon 136 to codon 167 of CT120A. The CT120B protein was predicted to have six transmembrane domains. In this study, we observed that the green fluorescent protein-tagged CT120B was localized on plasma membrane and in cytoplasm in SPC-A-1 cells. The expression of CT120B/A in normal lung tissue and in lung cancer cells was also examined. Results showed that the stable CT120B overexpression in SPC-A-1 cells resulted in a reduction of cell growth rate, and inhibited tumorigenecity and anchorage-independent growth in nude mice. The functions of CT120A and CT120B for cell growth appeared antagonistic. We suggested that the delayed G1/S phase transition might contribute to the inhibitory activities of CT120B on cell growth and that the deleted 32 amino acids missing in CT120B might be essential for the oncogenetic activities of CT120A.