Selenophosphate synthetase 1 (SEPHS1) belongs to the selenophosphate synthase 1 family, Class II subfamily. It has four different isoforms by alternative splicing. Isoform 1 and isoform 2 are gradually expressed during the cell cycle until G2/M phase and then decreased, which Isoform 3 is gradually expressed during the cell cycle until S phase and then decreased. SEPHS1 can be activated by phosphate ions and by potassium ions. It can synthesize synthesizes selenophosphate from selenide and ATP. Selenophosphate is the selenium donor used to synthesize selenocysteine, which is co-translationally incorporated into selenoproteins at in-frame UGA codons.