AIM: To investigate whether the side population (SP) cells possess cancer stem cell-like characteristics in vitro and the role of SP cells in tumorigenic process in gastric cancer. METHODS: We analyzed the presence of SP cells indifferent human gastric carcinoma cell lines, and then isolated and identified the SP cells from the KATO Ⅲ human gastric cancer cell line by flow cytometry. The clonogenic ability and self-renewal were evaluated by clone and sphere formation assays. The related genes were determined by reverse transcription polymerase chain reaction. To compare tumorigenic ability, SP and non-side population (NSP) cells from the KATO Ⅲ human gastric cancer cell line were subcutaneously injected into nude mice. RESULTS: SP cells from the total population accounted for 0.57% in KATO Ⅲ, 1.04% in Hs-746T, and 0.02% in AGS (CRL-1739). SP cells could grow clonally and have self-renewal capability in conditioned media. The expression of ABCG2, MDRI, Bmi-1 and Oct-4 was different between SP and NSP cells. However, there was no apparent difference between SP and NSP cells when they were injected into nude mice. CONCLUSION: SP cells have some cancer stem celllike characteristics in vitro and can be used for studying the tumorigenic process in gastric cancer. AIM: To investigate whether the side population (SP) cells possess cancer stem cell-like characteristics in vitro and the role of SP cells in tumorigenic process in gastric cancer. METHODS: We analyzed the presence of SP cells indifferent human gastric carcinoma cell lines, and then isolated and identified the SP cells from the KATO III human gastric cancer cell line by flow cytometry. The clonogenic ability and self-renewal were evaluated by clone and sphere formation assays. The related genes were determined by reverse transcription polymerase chain reaction. To compare tumorigenic ability, SP and non-side population (NSP) cells from the KATO III human gastric cancer cell line were subcutaneously injected into nude mice. RESULTS: SP cells from the total population accounted for 0.57% in KATO III, 1.04% in Hs -746T, and 0.02% in AGS (CRL-1739). SP cells could grow clonally and have self-renewal capability in conditioned media. The expression of ABCG2, MDRI, Bmi-1 and Oct- 4 was differen However, there was no apparent difference between SP and NSP cells when they were injected into nude mice. CONCLUSION: SP cells have some cancer stem celllike characteristics in vitro and can be used for studying the tumorigenic process in gastric cancer.