Transgene in the context of Intravital microscopy

In molecular cloning, a vector is any particle (e.g., plasmids, cosmids, Lambda phages) used as a vehicle to artificially carry a foreign nucleic sequence – usually DNA – into another cell, where it can be replicated and/or expressed. A vector containing foreign DNA is termed recombinant DNA. The four major types of vectors are plasmids, viral vectors, cosmids, and artificial chromosomes. Of these, the most commonly used vectors are plasmids. Common to all engineered vectors are the origin of replication, a multicloning site, and a selectable marker.

The vector itself generally carries a DNA sequence that consists of an insert (in this case the transgene) and a larger sequence that serves as the "backbone" of the vector. The purpose of a vector which transfers genetic information to another cell is typically to isolate, multiply, or express the insert in the target cell. All vectors may be used for cloning and are therefore cloning vectors, but there are also vectors designed specially for cloning, while others may be designed specifically for other purposes, such as transcription and protein expression. Vectors designed specifically for the expression of the transgene in the target cell are called expression vectors, and generally have a promoter sequence that drives the expression of the transgene. Simpler vectors called transcription vectors are only capable of being transcribed but not translated: they can be replicated in a target cell but not expressed, unlike expression vectors. Transcription vectors are used to amplify their insert.

Exogenous DNA is DNA originating outside the organism of concern or study. Exogenous DNA can be found naturally in the form of partially degraded fragments left over from dead cells. These DNA fragments may then become integrated into the chromosomes of nearby bacterial cells to undergo mutagenesis. This process of altering bacteria is known as transformation. Bacteria may also undergo artificial transformation through chemical and biological processes. The introduction of exogenous DNA into eukaryotic cells is known as transfection. Exogenous DNA can also be artificially inserted into the genome, which revolutionized the process of genetic modification in animals. By microinjecting an artificial transgene into the nucleus of an animal embryo, the exogenous DNA is allowed to merge the cell's existing DNA to create a genetically modified, transgenic animal. The creation of transgenic animals also leads into the study of altering sperm cells with exogenous DNA.

A viral vector is a modified virus designed to deliver genetic material into cells. This process can be performed inside an organism or in cell culture. Viral vectors have widespread applications in basic research, agriculture, and medicine.

Viruses have evolved specialized molecular mechanisms to transport their genomes into infected hosts, a process termed transduction. This capability has been exploited for use as viral vectors, which may integrate their genetic cargo—the transgene—into the host genome, although non-integrative vectors are also commonly used. In addition to agriculture and laboratory research, viral vectors are widely applied in gene therapy: as of 2022, all approved gene therapies were viral vector-based. Further, compared to traditional vaccines, the intracellular antigen expression enabled by viral vector vaccines offers more robust immune activation.

Gene delivery is the process of introducing foreign genetic material, such as DNA or RNA, into host cells. Gene delivery must reach the genome of the host cell to induce gene expression. Successful gene delivery requires the foreign gene delivery to remain stable within the host cell and can either integrate into the genome or replicate independently of it. This requires foreign DNA to be synthesized as part of a vector, which is designed to enter the desired host cell and deliver the transgene to that cell's genome. Vectors utilized as the method for gene delivery can be divided into two categories, recombinant viruses and synthetic vectors (viral and non-viral).

In complex multicellular eukaryotes (more specifically Weissmanists), if the transgene is incorporated into the host's germline cells, the resulting host cell can pass the transgene to its progeny. If the transgene is incorporated into somatic cells, the transgene will stay with the somatic cell line, and thus its host organism.