It is assumed that a cell that is transfected for any gene addition or replacement or was premarked with a cell tracker dye retains the characteristics of the original cell. The following experiments compare the original C6 rat glioma cell line with C6 cells transfected with the retroviral plasmid LacZ, and the human glioma cell lines GaMG, U373, U251, and D54 with cells stained with tracker dyes (Dil and DiO). We tested adhesion, migration and proliferation. C6 cell transfection did not affect adhesion but decreased (p < 0.05) migration. Dil staining resulted in a significant decrease (p < 0.01) in adhesion in all cell lines but U251. After DiO staining human cell lines U373 and D54 displayed a decrease in adhesion (p < 0.01) whereas U251 and GaMG cells had enhanced adhesion (p < 0.01). Dye marking of C6, GaMG and U373 cells did not alter migratory capacity. In contrast, Dil and DiO reduced migration of U251 and D54 cells (p < 0.05). There was a decrease (p < 0.01) in proliferation of the human cell lines after Dil staining. Transfection or membrane dyes can alter basic cell characteristics. The assumption that a transfected or dye marked cell is the same as the original cell but with an additional gene or the presence of a dye in the membrane is untenable.