Aims: Semaphorin 5A, a member of semaphorin family, was originally identified as axonal guidance factor functioning during neuronal development. Here, we investigated semaphorin 5A expression in gastric cancer and explored its roles in gastric carcinogenesis. Main methods: The expression of semaphorin 5A was examined by reverse transcription-polymerase chain reaction (RT-PCR) analysis in six gastric cancer cell lines and detected by real-time RT-PCR and Western blotting in 30 pairs of primary gastric cancer and normal gastric mucosa tissues. RNA interference (RNAi) technique was used to generate a semaphorin 5A-silenced stable cell line. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) and anchorage-in dependent growth as well as flow cytometry assays were conducted to examine the effect of siRNA-semaphorin 5A on gastric cancer cell growth, proliferation and apoptosis. Key findings: Semaphorin 5A was expressed in all human gastric cancer lines and the expression of semaphorin 5A was significantly higher in cancer tissues than that in normal mucosa tissues. siRNA-mediated semaphorin 5A knockdown significantly suppressed the proliferation and anchorage-independent growth, and induced the apoptosis of gastric cancer cell line SGC7901. Significance: The present study suggests that overexpression of semaphorin 5A may contribute to gastric carcinogenesis, which reveals a novel expression and function of semaphorin 5A outside the nervous system and adds more weight to our knowledge of semaphorin 5A. (C) 2009 Published by Elsevier Inc.