The biosynthetic pathway for selenocysteine (Sec), the 21st amino acid in the genetic code whose codeword is UGA, was recently determined in eukaryotes and archaea. Sec tRNA, designated tRNA SerSec , is initially aminoacylated with serine by seryl-tRNA synthetase and the resulting seryl moiety is converted to phosphoserine by O -phosphoseryl-tRNA kinase to form O -phosphoseryl-tRNA SerSec . Sec synthase (SecS) then uses O -phosphoseryl-tRNA SerSec and the active donor of selenium, selenophosphate, to form Sec-tRNA SerSec . Selenophosphate is synthesized from selenide and ATP by selenophosphate synthetase 2 (SPS2). Sec was the last protein amino acid in eukaryotes whose biosynthesis had not been established and the only known amino acid in eukaryotes whose biosynthesis occurs on its tRNA. Interestingly, sulfide can replace selenide to form thiophosphate in the SPS2-catalyzed reaction that can then react with O -phosphoseryl-tRNA SerSec in the presence of SecS to form cysteine-(Cys-)tRNA SerSec . This novel pathway of Cys biosynthesis results in Cys being decoded by UGA and replacing Sec in normally selenium-containing proteins (selenoproteins). The selenoprotein, thioredoxin reductase 1 (TR1), was isolated from cells in culture and from mouse liver for analysis of Cys/Sec replacement by MS. The level of Cys/Sec replacement in TR1 was proportional to the level of selenium in the diet of the mice. Elucidation of the biosynthesis of Sec and Sec/Cys replacement provides novel ways of regulating selenoprotein functions and ultimately better understanding of the biological roles of dietary selenium.