To express a cloned gene (for a desired product) in a transformed cell means that it has to transcript and translate. Sometimes functional proteins are produced by post-translation modifications. Protein synthesis is based on the encoded information in the sequence of the DNA strand bases. Structural gene directs the cells machinery for sequential linear arrangements of linear amino acid units to form a particular protein. Base sequences lying adjacent to the structural genes, control expression.

In prokaryotes and eukaryotes, the mechanism of transcription and translation is the same, but control signals are different. Many eukaryotic genes having coding and non-coding sequences (exons/introns), were transcribed (but not expressed). This mRNA processed, before its release from nucleus, is made up of exons. Proper expression of a cloned eukaryotic gene in bacteria can be done only if the gene lacks introns. The generation of cDNA to remove introns would also remove the upstream sites.

Also, the eukaryotic transcript would not carry rbs; therefore, eukaryotic genes should be supplied at minimum a prokaryotic promoter and rbs for expression. Many prokaryotic operon genes are organised in inducible or repressible groups of related genes whose expression is regulated co-ordinately. The controlling element in the expression of these groups is the operator, which is a sequence between the start of the gene and the promoter. When expression is turned off, a specific repressor protein binds to the operator site and prevents transcription.