Abstract: This invention concerns a method for recovering mutant cells which produce a substance in a quantifiably reduced amount relative to wild type cells, and the mutant cells so recovered. The method involves contacting the mutant cells under suitable conditions with a suitable amount of an appropriate detectable compound capable of binding to the substance when it is present in the cells so as to permit the detectable compound to bind to the substance. The amount of the detectable compound bound to the substance in the cells is then detected, and the cells which produce the substance in the quantifiably reduced amount are thereby detected. Such cells which produce the quantifiably reduced amount of the substance are then recovered. This invention is applicable to a wide variety of cell types of various genetic characteristics and should therefore be useful in providing a wide range of useful mutant cells.