In medicine
Medicine is the science and art of healing. It encompasses a variety of health care practices evolved to maintain and restore health by the prevention and treatment of illness....

 a heterologous transplant means 'between species' or 'from one species to another'.

In cell biology
Cell biology
Cell biology is a scientific discipline that studies cells – their physiological properties, their structure, the organelles they contain, interactions with their environment, their life cycle, division and death. This is done both on a microscopic and molecular level...

 and protein biochemistry
Protein methods
Protein methods are the techniques used to study proteins.There are genetic methods for studying proteins, methods for detecting proteins, methods for isolating and purifying proteins and other methods for characterizing the structure and function of proteins, often requiring that the protein first...

, heterologous expression means that a protein is experimentally put into a cell that does not normally make (i.e., express
Gene expression
Gene expression is the process by which information from a gene is used in the synthesis of a functional gene product. These products are often proteins, but in non-protein coding genes such as ribosomal RNA , transfer RNA or small nuclear RNA genes, the product is a functional RNA...

) that protein. Heterologous (meaning 'derived from a different organism') refers to the fact that often the transferred protein was initially cloned from or derived from a different cell type or a different species from the recipient. Typically the protein itself is not transferred, but instead the 'correctly edited' genetic material coding for the protein (the complementary DNA or cDNA) is added to the recipient cell. The genetic material that is transferred typically must be within a format that encourages the recipient cell to express the cDNA as a protein (i.e., it is put in an expression vector
Expression vector
An expression vector, otherwise known as an expression construct, is generally a plasmid that is used to introduce a specific gene into a target cell. Once the expression vector is inside the cell, the protein that is encoded by the gene is produced by the cellular-transcription and translation...

). Methods for transferring foreign genetic material into a recipient cell include transfection
Transfection is the process of deliberately introducing nucleic acids into cells. The term is used notably for non-viral methods in eukaryotic cells...

 and transduction
Transduction is a mechanism whereby genetic material may be transferred from the genes of a bacterium to another bacterium. This may include the actual covalent-bonding of new genetic markers...

. The choice of recipient cell type is often based on an experimental need to examine the protein's function in detail, and the most prevalent recipients, known as heterologous expression systems, are chosen usually because they are easy to transfer DNA into or because they allow for a simpler assessment of the protein's function.

In structural biology
Structural biology
Structural biology is a branch of molecular biology, biochemistry, and biophysics concerned with the molecular structure of biological macromolecules, especially proteins and nucleic acids, how they acquire the structures they have, and how alterations in their structures affect their function...

 a heterologous association is a binding mode between the protomer
In structural biology, a protomer is the structural unit of an oligomeric protein. A protomer can be a protein subunit or several different subunits, that assemble in a defined stoichiometry to form an oligomer. The protomer is the smallest subset of different subunits that form the oligomer. The...

s of a protein structure. In a heterologous association, each promoter contributes a different set of residues to the binding interface. In contrast, two protomers form an isologous association when they contribute the same set of residues to the protomer-protomer interface.
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