Nucleofection is a method to transfer substrates into mammalian cells so far considered difficult or even impossible to transfect. Examples for such substrates are nucleic acids, like the DNA of an isolated gene cloned into a plasmid, or small interfering RNA (siRNA) for knocking down expression of a specific endogenous gene.
Primary cells, for example stem cells, especially fall into this category, although many other cell lines are also difficult to transfect. Primary cells are freshly isolated from body tissue and thus cells are unchanged, closely resembling the in-vivo situation, and are therefore of particular relevance for medical research purposes. In contrast, cell lines have often been cultured for decades and may significantly differ from their origin.