Introduction of NEU3 cDNA into N1 cells resulted in increased cell growth. (A) NEU3 mRNA levels in SK‐MEL‐28 and a mutant line, SK‐MEL‐28‐N1. (B) NEU3 sialidase activities in SK‐MEL‐28 and a mutant line, SK‐MEL‐28‐N1, measured using gangliosides as substrates. Both mRNA and enzyme activity of NEU3 were equivalent between the parent line and the mutant line. (C) NEU3 mRNA levels in three NEU3‐transfectant lines and a control. Relative mRNA levels as analyzed using q‐RT‐PCR are shown. (D) Sialidase activities in the same sets of cell lines. Sialidase activity was analyzed using GM3 as a substrate. NEU3 activities in the transfectants (N1/neu3‐1, ‐54, ‐65) and a control (N1/Vec) are shown. (E) Thin‐layer chromatography of extracted acidic fractions (left) and neutral fractions (right) from a vector control (vec104) and a NEU3 transfectant (neu3‐54). CDH, lactosylceramide. (F) Cell proliferation of the NEU3 transfectants was compared with that of the controls (N1/vec: 9, 104) using a MTT assay. Absorption of the transfectants were significantly higher than that of the controls at day 5 (*P < 0.05). Any transfectant showed significantly higher absorption than any control with P < 0.005. Similar results were repeatedly obtained in several experiments.