In its most precise usage, the term transgene describes a segment of DNA containing a gene sequence that has been isolated from one organism and is introduced into a different organism. This non-native segment of DNA may retain the ability to produce RNA or protein in the transgenic organism, or it may alter the normal function of the transgenic organism's genetic code. In general, the DNA is incorporated into the organism's germ line. For example, in higher vertebrates this can be accomplished by injecting the foreign DNA into the nucleus of a fertilized ovum. This technique is routinely used to introduce human disease genes or other genes of interest into strains of laboratory mice to study the function or pathology involved with that particular gene.
In looser usage, transgene can describe any DNA sequence, regardless of whether it contains a gene coding sequence or it has been artificially constructed, which has been introduced into an organism or vector construct in which it was previously not found.
In practical terms, a transgene can be either a cDNA (complementary DNA) segment, which is a copy of mRNA (messenger RNA), or the gene itself residing in its original region of genomic DNA. The difference between these two lies in the fact that the cDNA has been processed to remove introns and also, usually, does not include the regulatory signals that are embedded around and in the gene. The advent of annotated cloned regions of the genome alongside the genome sequence, in particular as large clones in BACs (bacterial artificial chromosomes) or fosmids, and recombineering, which is the method that permits the engineering of these large clones, has changed the practice of transgenesis from its origins with cDNA-based constructs towards the more reliable genomic-based constructs.