Opti-MEM I Reduced Serum Medium is a serum-free medium. The reason why it is called reduced-serum medium is because you only need to supplement this medium with 1-5% FBS instead of the standard 10% FBS.
Opti-MEM I Reduced Serum Medium can be used as is (without serum) for transfection of cells. However, for long-term cell culture, you need to add serum to the medium to keep cells healthy.
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