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The ideal cells are between passages 5-20 after thawing and growing in log phase. Prior to transfection, cell viability should be greater than 90% and 70~80% confluent. For transfection, we recommend using Opti-MEM (serum-free) for lipid/payload complex formation and culture medium supplemented with 10% serum. Lower amount of serum (<10%) works as well but post-transfection (4-24 hr) media change with complete growth media is recommended.