U is Selenocysteine

The letter U represents Selenocysteine. This is an animo acid bead that can be strung on a necklace or bracelet. Selenocysteine is present in several enzymes (for example glutathione peroxidases, glycine reductases, some hydrogenases, etc.). Unlike other amino acids it is not coded for directly in the gene, but rather in an insertion element immediately after the stop codon. The resulting mRNA sequence mates with a specialized Sslenocysteine tRNA during translation. IUPAC/IUBMB has officially recommended the three-letter symbol Sec and the one-letter symbol U for selenocysteine.