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 a nucleic acid, most frequently plasmid DNA encoding an expression cassette, has been introduced into eukaryotic cells. 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 human, 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.
Some fungi, like yeasts or zygomycota, are susceptible to transformation by dedifferentiating their hyphae to protoplasts and adding Cl2Ca 10–50 mM, Tris-HCl 10 mM, polyethylene glycol (40%) and the DNA construct (> 5µg/ml) to a 10^8 protoplast/ml solution. Under this pH condition, PEG acts as a binder, promoting protoplasts to clump together and trap introduced DNA strands. This process has a relatively high efficiency, despite the fact that eukaryotic cells have abundant exonucleases that degrade exogenous DNA. 