The novel DNA which is introduced into cells can take many forms. In the case where it consists of one or more genes which encode novel proteins, these are often referred to as transgenes. To be expressed, the novel gene will need to be under the control of a promoter. This will determine the level to which it is expressed, and can also determine when during development and where in the organism it is expressed. For an organism to be truly transgenic, it has to contain a transgene in all of its cells (Figure 12.1). This means that the gene must be stably replicated along with the host DNA in every cell cycle for all cells (Figure 12.2). This is achieved in practice by integrating the novel DNA into one of the chromosomes of the host organism. This integration may be random or, in some cases, directed to a particular place on the chromosome.