Cytosine-phosphate-guanosine nucleotides (CpG) in eukaryotic genomes are often methylated by DNA methyltransferases. In bacteria, this mechanism is used for delimitation of gene products. A negative consequence of such methylation can be a reduced expression rate of the affected gene. Avoiding such motifs in the vector system, e.g. by the use of minicircle DNA, is therefore advantageous.