Systems for the removal of a selection marker and their combination with apositive marker

Many systems have been developed for the removal of a selection marker in o rder to generate marker-free transgenic plants. These systems consist of (1 ) a site-specific recombination system (Cre/lox) or a phage-attachment regi on (attP) to remove the selectable marker gene and (2) a transposable eleme nt system (Ac) or a cotransformation system to segregate the gene of intere st from the selectable marker gene. Overall, the process is more time-consu ming than conventional transformation methods because two rounds of transfo rmation - two steps of regeneration or sexual crossings - are required to o btain the desired transgenic plants. Recently, removal systems combined wit h a positive marker, denoted as MAT vectors, have been developed to save ti me and effort by generating marker-free transgenic plants through a single- step transformation. We summarize here the transformation procedures using these systems and discuss their feasibility for practical use.