The synthetic cycle of selenocysteine (Sec) synthesis starts with misacylation (serylation) of the cognate transfer RNA (tRNA) for Sec, tRNASec, with serine (Ser) by Serine--tRNA ligase (SARS) homodimer. The structural homology of tRNASec and tRNASer enables SARS to form Ser-tRNASec (Vincent et al. 1997, Amberg et al. 1996, Heckl et al. 1998).