The Expresso Single-Shot and Double-Shot HEK-293, HeLa, CHO-K1, NIH-3T3, COS-1, and COS-7 Mammalian Cells are pre-counted, pre-aliquoted versions of their standard counterpart cell lines. However, they have been frozen in a state of suspended high efficiency. This means that all you need to do is thaw them, plate them, and transfect them 3 hours later. This time savings allows you to transfect at a moments notice, without having to passage your cells for a day or longer prior to transfecting. Additionally, Expresso cells provide higher transfection efficiency than the standard cell lines when using most transfection reagents.