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In the past, 293 cells were the overwhelming choice of researchers performing transient protein expression because 293 cells could generate significantly higher titers than CHO cells in a transient setting. For example, Expi293 is capable of generating titers greater than 50-fold higher than the FreeStyle CHO™ transient expression system. With the introduction of the ExpiCHO transient expression system, this is no longer the case, and indeed in many/most instances ExpiCHO is significantly higher expressing than Expi293. Secondly, since CHO cells are used to manufacture >70% of current licensed therapeutic proteins today, the ability to start drug development work in CHO cells and stay in CHO cells from discovery through bioproduction reduces risk, as transient CHO-derived proteins are more similar in terms of critical quality attributes to proteins made in stable CHO cells. Thirdly, the ExpiCHO expression system offers unparalleled flexibility in terms of protocol options. Three different protocols are provided in the ExpiCHO manual, Standard, High Titer and Max Titer protocols, which allow researchers to choose the option that best fits their needs in terms of equipment, time, number of steps and amount of protein desired. Additionally, because of the robustness of the ExpiCHO system, there are many ways that the system can be further modified to meet the particular needs of a given lab.