In the photolithographic approach, there is no exposure to either strongly acidic or basic conditions, except in the final chemical deprotection step, with ethylenediamine in ethanol (1:1), that results in the removal of the exocyclic amine protecting groups on the adenine, cytosine and guanine nucleobases, as well as the β cyanoethyl protecting group on the phosphodiester. With the use of 4,5-dicyanoimidazole (DCI) as the activator, we have found that intermittent oxidation, or even a single final oxidation, is sufficient in the context of photolithographic synthesis . However, the use of more acidic activators such as 5-ethylthio-1H-tetrazole (ETT) , 5-benzylthio-1H-tetrazole (BTT) , and 5-[3,5-bis(trifluoro-methyl)-phenyl]-1H-tetrazole (Activator 42) , requires oxidation in each cycle to prevent cleavage at the phosphite triester bond .