From studies with their cDNA, the types 1 and 3 deiodinases (D1 and D3) have been shown unequivocally to be selenoproteins. Studies with recently cloned cDNAs for the mammalian type 2 deiodinase (D2) indicate that they also code for selenoproteins. However, these D2 cDNAs are not full length and they do not contain an essential selenocysteine insertion sequence (SECIS) in their 3′UTR; a heterologous SECIS had to be ligated to the coding region before expression of the D2 could be achieved. Thus their role as cDNAs for the native D2 is open to question. We now report the cloning of a 5.8 kb cDNA for the mouse D2. This cDNA contains a SECIS in its 3′UTR located more than 4.5 kb from the coding region. When the mRNA transcribed in vitro from this cDNA is injected into X. laevis oocytes, a deiodinase with characteristics of D2 is expressed.