Transient expression

Transient expression, more frequently referred to "transient gene expression", is the temporary expression of genes that are expressed for a short time after nucleic acid, most frequently plasmid DNA encoding an expression cassette, has been introduced into eukaryotic cells with a chemical delivery agent like calcium phosphate (CaPi) or polyethyleneimine (PEI). However, unlike "stable expression," the foreign DNA does not fuse with the host cell DNA, resulting in the inevitable loss of the vector after several cell replication cycles. The majority of transient gene expressions are done with cultivated animal cells. The technique is also used in plant cells; however, the transfer of nucleic acids into these cells requires different methods than those with animal cells. In both plants and animals, transient expression should result in a time-limited use of transferred nucleic acids, since any long-term expression would be called "stable expression."

Methodology varies depending on the organism to transform. While plants can be transformed with a construct introduced into Agrobacterium tumefaciens via agroinfiltration or floral dip, most animal cells would require a viral vector. In humans, the field of transient transformation advanced rapidly during the 2020–2021 COVID-19 pandemic with major COVID-19 vaccines using either direct mRNA transfer into human or adenovirus vectors, with the RNA being expressed in the host human to produce spike proteins that induce an immune response.