Selenocysteine is recognized as the 21st amino acid but, unlike other amino acids present in proteins, it is not coded for directly in the genetic code. Selenocysteine is encoded by a UGA codon, which is normally a stop codon; however, like the other amino acids used by cells, selenocysteine has a specialized transfer RNA (tRNA). Selenocysteine was named as an amino acid in 2002 and found to be the selenium analogue of cysteine, in which a selenium atom replaces sulfur. Selenocysteine is present in several enzymes, such as formate dehydrogenases, glycine reductases, and some hydrogenases. It has been discovered that HIV-1 encodes a functional selenoprotein, and patients with HIV infection have been shown to have a lower-than-average blood plasma selenium level.
| Micromole per liter | µmol/L = mcmol/L = umol/L = µM/L = mcM/L = uM/L = micromol/L |
| Milligram per liter | mg/L = millig/L = milligram/L = mg/liter |
| Milligram per deciliter | mg/dL = millig/dL = milligram/dL |
| Milligram per 100 milliliters | mg/100mL = millig/100mL = milligram/100mL |
| Milligram percent | mg% = millig% = milligram% |
| Microgram per milliliter | µg/mL = mcg/mL = ug/mL = microg/mL = microgram/mL |