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.

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 conditions, 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.

• Yeast Integrating plasmids (YIp): These plasmids lack an ORI and must be integrated directly into the host chromosome via homologous recombination.
• Yeast Replicating plasmids (YRp):  These vectors contain an Autonomously Replicating Sequence (ARS) derived from the yeast chromosome. As the name suggests, these vectors can replicate independently of the yeast chromosome; however, they tend to be unstable and may be lost during budding.
• Yeast Centromere plasmids (YCp): These are considered low copy vectors and incorporate part of an ARS along with part of a centromere sequence (CEN). These vectors replicate as though they are small independent chromosomes and are thus typically found as a single copy. Unlike the ARS vectors, CEN vectors are stable without integration.
• Yeast Episomal plasmids (YEp): These are most similar to bacterial plasmids and are considered “high copy”. A fragment from the 2 micron circle (a natural yeast plasmid) allows for 50+ copies to stably propogate per cell. The copy number of these vectors can also be controlled if specific regulatable elements are included. The episomal vector is not available for filamentous fungi.