Selenoprotein synthesis is conserved from bacteria to man. It involves the 
differential decoding of the UGA stop codon as selenocysteine. The proteomes of 
both prokaryotes and eukaryotes, with the exception of yeast, contain only few 
selenoproteins. This low number is explained by a counterselection of readily 
oxidized selenocysteine after the introduction of oxygen into the atmosphere and 
the need to conserve selenoenzymes that control redox homeostasis of cells. Lack 
of selenoprotein synthesis in vertebrates impairs the oxidative stress defence 
and causes lethality. Here we show that Drosophila mutants that lack the 
translation elongation factor SelB/eEFsec fail to decode the UGA codon as 
selenocysteine, but they are viable and fertile. Oxidative stress responses and 
the lifespan of these flies are not affected. Protecting cells from oxidative 
stress can therefore not account for the selection pressure that conserves 
selenoprotein biosynthesis during the course of evolution.