Selenoproteins are a diverse group of proteins that contain selenocysteine 
(Sec), the 21st amino acid. In the genetic code, UGA serves as a termination 
signal and a Sec codon. This dual role has precluded the automatic annotation of 
selenoproteins. Recent advances in the computational identification of 
selenoprotein genes have provided a first glimpse of the size, functions, and 
phylogenetic diversity of eukaryotic selenoproteomes. Here, we describe the 
identification of a selenoprotein family named SelJ. In contrast to known 
selenoproteins, SelJ appears to be restricted to actinopterygian fishes and sea 
urchin, with Cys homologues only found in cnidarians. SelJ shows significant 
similarity to the jellyfish J1-crystallins and with them constitutes a distinct 
subfamily within the large family of ADP-ribosylation enzymes. Consistent with 
its potential role as a structural crystallin, SelJ has preferential and 
homogeneous expression in the eye lens in early stages of zebrafish development. 
A structural role for SelJ would be in contrast to the majority of known 
selenoenzymes. The unusually highly restricted phylogenetic distribution of 
SelJ, its specialization, and the comparative analysis of eukaryotic 
selenoproteomes reveal the diversity and functional plasticity of selenoproteins 
and point to a mosaic evolution of the use of Sec in proteins.