Heterologous expression refers to the expression of a gene or part of a gene in a host organism, which does not naturally have this gene or gene fragment. Insertion of the gene in the heterologous host is performed by recombinant DNA technology. After being inserted in the host, the gene may be integrated into the host DNA, causing permanent expression, or not integrated, causing transient expression. Heterologous expression can be done in many type 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. 
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