During a perfusion process, the product is constantly removed from the bioreactor while fresh media is fed. In contrast, in a fed-batch process the product stays over 14-17 days in a bioreactor at cell cultivation conditions (37 °C, high shear forces). If products are unstable over fed-batch cultivation time, perfusion processes might be the process of choice as harvested supernatant can be directly stored at the required storage conditions. Apart from the fact of instability, posttranslational modifications can be kept constant over the time period the perfusion process is run by continuously holding cells in a constant state.