/sis"teen, -tin/ , n. Biochem.
a crystalline amino acid, C 6 H 12 O 4 N 2 S 2 , occurring in most proteins, esp. the keratins in hair, wool, and horn, and yielding cysteine on reduction.
[ 1835-45; CYST- + -INE 2 ; so called because found in the bladder ]