We subsequently tested our selection conditions by mixing a subpopulation of GRLMR expressing cells into a culture containing 10-fold more cells expressing inactivated GRLMR, as shown in Fig 4D . At day 3, a portion of the culture was plated on LB-Agar. From the LB-Agar, 10 colonies were grown in liquid culture, from which plasmids were purified and sequenced. This sequencing revealed that all 10 isolates contained only active GRLMR sequence. This shows that in the selection condition (40mM SeO3 2-), GRLMR expressing cells outcompete those cells that express inactivated GRLMR. This condition, then, is expected to put selective pressure on cells containing a GRLMR mutant library, favoring those cells with more effective Se-reducing GRLMR variants.