Most of these selenoproteins were redox proteins, which used Sec either to coordinate redox-active metals or for thiol/disulfide-based redox catalysis. Moreover, among 24 selenoprotein families detected in the symbionts’ metagenomic dataset, at least 17 (67 sequences, 81.7%) were homologs of known thiol oxidoreductases or possessed Trx-like fold (Table 1). Many of these selenoproteins contained a conserved UxxC/UxxS/CxxU/TxxU redox motif.