The non-recombinants are separated from recombinants; i.e., a r-DNA is introduced in bacteria, some bacteria are successfully transformed some remain non-transformed. When grown on medium containing ampicillin, bacteria die due to lack of ampicillin resistance. The position is later noted on nitrocellulose paper and separated out to move them to nutrient medium for mass production of required product.
An alternative to a selectable marker is a screenable marker which can also be denoted as a reporter gene, which allows the researcher to distinguish between wanted and unwanted cells, e.g. between blue and white colonies. These wanted or unwanted cells are simply un-transformed cells that were unable to take up the gene during the experiment.