Positive selection based on glucides

The general method of positive selection could use any growth-promoting catabolite precursor, and was first illustrated using various sugars that do not promote growth until altered by an enzyme for which the gene was the transformation marker.  The first patents were opposed by many parties on the basis that the idea had been described sufficiently prior to the patent filing that the grant of a monopoly to a different party was not justified, but after many appeals and some amendments these patents are still in force.