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Heterologous expression refers to the expression of a gene or part of a gene in a host organism that does not naturally have this gene or gene fragment. Insertion of the gene in the heterologous host is performed by recombinant DNA technology.

Depending on the duration of recombination in the host genome, there are two types of heterologous expression. They are long-term and short-term (temporarily, stable) respectively. Long-term is overexpression of gene and short-term is a temporary modification made to a gene which lasts for a day to 3 days.

1. Techniques to isolate specified genes:Gene identification using computer-based methods is known as a heterologous screening technique(1). However, due to faulty membrane protein purification, this approach makes it difficult to separate genes in every aspect. Another approach to isolating membrane proteins uses electrophoresis, a technique known as radiolabeled ligand or inhibitors is used(2).
2. Techniques to incorporate genes into the host: Gene gun delivery, Electroporation, Viral transduction, and Lipofection.

Gene gun delivery is a method mostly used in plants, which works on the principle of the bombardment of genes coated with metal particles.

Electroporation is a method that uses high voltage to create pores in the cell membranes of mammalian cells.

Viral transduction is a method that uses viral vectors and is used for the stable introduction of genes into the target cells.

Lipofection is a method used to inject the gene with the help of liposomes.

After being inserted in the host, the gene may be integrated into the host DNA, causing permanent expression, or not integrated, causing transient expression. A heterologous expression can be done in many types of host organisms. The host organism can be a bacterium, yeast, mammalian cell, or plant cell. This host is called the "expression system". Homologous expression, on the other hand, refers to the overexpression of a gene in a system from where it originates.

Genes are subjected to heterologous expression often to study specific protein interactions. E. coli, yeast (S. cerevisiae, P. pastoris), immortalized mammalian cells, and amphibian oocytes (i.e. unfertilized eggs) are commonly for studies that require heterologous expression.