Towards the ideal GMP: homologous recombination and marker gene excision.

A mayor aim of biotechnology is the establishment of techniques for the precise manipulation of plant genomes. Two major efforts have been undertaken over the last dozen years, one to set up techniques for site-specific alteration of the plant genome via homologous recombination ("gene targeting") and the other for the removal of selectable marker genes from transgenic plants. Unfortunately, despite multiple promising approaches that will be shortly described in this review no feasible gene targeting technique has been developed till now for crop plants. In contrast, several alternative procedures have been established successfully to remove selectable markers from plant genomes. Intriguingly besides techniques relying on transposons and site-specific recombinases, recent results indicate that homologous recombination might be a valuable alternative for the excision of marker genes.