Concerns have been raised that the presence of selectable marker genes such as antibiotic or herbicide resistant genes might be unpredictable hazard to the ecosystem as well as to human health. The genes coding enzymes that catalyze special sugars have showed big practical potency in plant transformation as safe marker genes. This kind of sugar catabolic maker genes can give plant cells the ability to use selective agent sugars such as xylose or mannose, therefore the transformed cells could get sufficient energy and grow dominantly while the untransformed cells are starved and inhibited from growing but not be killed. It is called positive selection system. Now the safe marker genes such as xylA（xylose isomerase gene） and pmi（phosphomannose isomerase gene） have been successfully used in plant transformation.