Human renal cell carcinoma (RCC) has been characterized by remarkable changes in ganglioside composition. TOS1 cells, typical of metastatic RCC, are characterized by predominance of GM2 as monosialoganglioside, and beta 1,4GalNAc disialyl-Lc(4) (RM2 antigen) as disialoganglioside [J. Biol. Chem. 276 (2001) 16695]. In order to observe the functional role of gangliosides in RCC malignancy, TOS1 cells were transfected with short interfering RNA (siRNA) based on open reading frame sequence of beta 1,4GalNAc transferase (beta 1,4GalNAc-T), and its disordered sequence of siRNA (dsiRNA) as control. In siRNA transfectant, beta 1,4GalNAc-T mRNA level and GM2 expression were greatly reduced, whereby GM3 expression appeared. In contrast, RM2 antigen level was unchanged, even though it has the same beta 1,4GalNAc epitope at the terminus. dsiRNA transfectant showed no change of beta 1,4GalNAc-T mRNA and did not express GM3. Concomitant with reduction of GM2 and appearance of GM3, siRNA transfectant showed greatly reduced motility and invasiveness, although growth rate was unaltered. Both transfectants with siRNA and dsiRNA expressed the same level of tetraspanin CD9. Since CD9/GM3 complex is known to reduce integrin-dependent motility and invasiveness [Biochemistry 40 (2001) 6414], it is plausible that motility and invasiveness of siRNA transfectant of TOS1 cells may be reduced by enhanced formation of such complex.