Error-catching mechanism within the IIS-alphoidtetO-HAC system. Recombination reactions mediated by Cre, ΦC31, and ΦBT1 do not always go to completion. Because maintenance, storage, and screening of vertebrate cell colonies is very labor-intensive compared to that for bacteria or yeast colonies, an error catching system was designed into the IIS to lighten the workload. (a-b) Products of incomplete recombination reactions involving the Type I carrier vector A167 retain an actively expressed GHT marker. (c-d) Products of incomplete recombination reactions between the Type II carrier vector A169 retain an expressed PCF marker. The selection agents to remove each misassembled product are listed. This figure was adapted from (Lee et al., 2018).