IUPAC Name: (2R)-2-Amino-3-[(2R)-2-amino-3-hydroxy-3-oxopropyl]diselanylpropanoic acid
Synonyms: (2R,2'R)-3,3'-Diselane-1,2-diylbis(2-aminopropanoic acid) (non-preferred name) ; (R,R)-3,3'-Diseleno-bis(2-aminopropionic acid)
; Alanine, 3,3'-diselenodi-, L- ; DL-alanine, 3,3'-diselenobis- ; L-3,3'-Diselenodialanine ; L-selenocystine ; Seleno-L-cystine
CAS NO: 29621-88-3
Molecular Formula of l-Selenocystine (CAS NO.29621-88-3) : C₆H₁₂N₂O₄Se₂
Molecular Weight of l-Selenocystine (CAS NO.29621-88-3) : 334.09
Molecular Structure of l-Selenocystine (CAS NO.29621-88-3) :
EINECS: 275-728-7
Mol File: 29621-88-3.mol
Flash Point: 279.3 °C
Enthalpy of Vaporization: 89.19 kJ/mol
Boiling Point: 538.3 °C at 760 mmHg
Vapour Pressure: 5.12E-13 mmHg at 25°C
Melting point: 224.5-229.5 °C(lit.)
Storage temp. 2-8°C
Appearance:yellow fine powder

A poison by intraperitoneal route. When heated to decomposition it emits toxic vapors of NO_x and Se.
Hazard CodesT,N
Risk Statements 23/25-33-50/53
R23/25:Toxic by inhalation and if swallowed. 
R33:Danger of cumulative effects. 
R50/53:Very toxic to aquatic organisms, may cause long-term adverse effects in the aquatic environment.
Safety Statements 20-21-28-45-60-61
S20:When using, do not eat or drink. 
S21:When using do not smoke. 
S28:After contact with skin, wash immediately with plenty of soap-suds. 
S45:In case of accident or if you feel unwell, seek medical advice immediately (show the label whenever possible.) 
S60:This material and its container must be disposed of as hazardous waste. 
S61:Avoid release to the environment. Refer to special instructions / safety data sheets.
RIDADR UN 3283 6.1/PG 3
WGK Germany 3
RTECS AY6032000

ACGIH TLV: TWA 0.2 mg(Se)/m³