The ExpiCHO and Expi293 transient expression systems offer all-in-one solutions for generating high-titer recombinant proteins for a broad range of research applications including candidate drug identification, reagent production, structural biology and vaccine research and membrane protein biology.  While both Expi systems offer the ability to generate high levels of recombinant proteins, the inherent differences between HEK293 cells and CHO cells make these systems differentially applicable for various protein expression needs.  Here, we present the latest data on the ExpiCHO and Expi293 Expression systems as well as suggested paradigms for instances where either Expi293 or ExpiCHO would be most applicable for a given research requirement.