Multiple approaches have been developed for the removal of marker genes from the plastid genome. The most efficient system uses the P1 phage CRE-loxP site-specific recombination system (Figure 7). According to the CRE-loxP scheme, the marker gene and the gene of interest are introduced into the plastid genome in the absence of CRE activity. The marker gene is flanked by two directly oriented lox sites. When elimination of the marker gene is required, a gene encoding a plastid-targeted CRE site-specific recombinase is introduced into the nucleus which, subsequent to its import in plastids, excises sequences between the loxP sites. In tobacco, removal of the marker gene extends the time needed to obtain marker-free plants only by 1 month.