ˈsiˌstēn, -_stə̇n noun
( -s )
Etymology: cyst- + -ine; fr its discovery in urinary calculi
: a colorless crystalline amino acid [−SCH 2 CH(NH 2 )COOH] 2 occurring as a constituent of most proteins (as the keratins in hair, wool, and horn) from which it can be obtained by hydrolysis and yielding cysteine on reduction; β,β′-dithio-di-alanine