Historically, transient and stable transfections have been performed for different purposes. Transient transfections can rapidly provide reasonable amounts of recombinant proteins (mg quantities) for early evaluation of their activity and typically transient transfections were used in the screening stages of a project when potential clinical candidates were being selected. Due to the time that it took to generate stable clonal cell lines (7–12 months), they were generated only when clinical candidates had been selected. Over the past 20 years, however, technological advances in the generation of stable and transient transfectants have improved productivity and reduced timelines, leading to some debate over whether it is better to use transiently or stably transfected cell lines during development.