What are the limitations of the mammalian protein expression system?

The main disadvantage is the high cost of protein production, due to slow cell growth. Expensive media and culture conditions are required to fuel the necessary carbon dioxide supply, as well as expensive transfection reagents. Another limitation is that high protein yield can only be obtained by suspension cultures (e.g. CHO cells). Also, these expression systems take longer to be carried out because mammalian cells have longer doubling times and may take several days to reach the desired cell density for protein expression. These systems are used for generating large amounts of protein within 1 to 2 weeks.