RNA can also be transfected into cells to transiently express its coded protein, or to study RNA decay kinetics. RNA transfection is often used in primary cells that do not divide.
siRNAs can also be transfected to achieve RNA silencing (i.e. loss of RNA and protein from the targeted gene). This has become a major application in research to achieve "knock-down" of proteins of interests (e.g. Endothelin-1) with potential applications in gene therapy. Limitation of the silencing approach are the toxicity of the transfection for cells and potential "off-target" effects on the expression of other genes/proteins.
RNA can be purified from cells after lysis or synthesized from free nucleotides either chemically, or enzymatically using an RNA polymerase to transcribe a DNA template.  As with DNA, RNA can be delivered to cells by a variety of means including microinjection, electroporation, and lipid-mediated transfection.  If the RNA encodes a protein, transfected cells may translate the RNA into the encoded protein.  If the RNA is a regulatory RNA (such as a miRNA), the RNA may cause other changes in the cell (such as RNAi-mediated knockdown).
Encapsulating the RNA molecule in lipid nanoparticles was a breakthrough for producing viable RNA vaccines, solving a number of key technical barriers in delivering the RNA molecule into the human cell.
RNA molecules shorter than about 25nt (nucleotides) largely evade detection by the innate immune system, which is triggered by longer RNA molecules.  Most cells of the body express proteins of the innate immune system, and upon exposure to exogenous long RNA molecules, these proteins initiate signaling cascades that result in inflammation.  This inflammation hypersensitizes the exposed cell and nearby cells to subsequent exposure.  As a result, while a cell can be repeatedly transfected with short RNA with few non-specific effects, repeatedly transfecting cells with even a small amount of long RNA can cause cell death unless measures are taken to suppress or evade the innate immune system (see "Long-RNA transfection" below).
Short-RNA transfection is routinely used in biological research to knock down the expression of a protein of interest (using siRNA) or to express or block the activity of a miRNA (using short RNA that acts independently of the cells RNAi machinery, and therefore is not referred to as siRNA).  While DNA-based vectors (viruses, plasmids) that encode a short RNA molecule can also be used, short-RNA transfection does not risk modification of the cells DNA, a characteristic that has led to the development of short RNA as a new class of macromolecular drugs.
Long-RNA transfection is the process of deliberately introducing RNA molecules longer than about 25nt into living cells.  A distinction is made between short- and long-RNA transfection because exogenous long RNA molecules elicit an innate immune response in cells that can cause a variety of nonspecific effects including translation block, cell-cycle arrest, and apoptosis.