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Cell aggregation is influenced by a variety of factors, including basal culture medium, media additives, culture vessels and agitation rates. It is also important to note that certain cell strains are more prone to clumping than others. Screening and selection of the right culture medium is a good way to reduce cell aggregation. Adding surfactants, such as pluronic F-68 to the culture can also help to alleviate aggregation. Continuous culturing of cells with non-aggregated cells is another way to minimize cell aggregation issues. PEI should not be a major factor for cell aggregation. However, not all culture media and additives are suitable for PEI transfection. Thus, for cells that are prone to aggregation, it may be better to use a transfection method, such as electroporation, that is compatible with many different types of culture media and conditions.