A compound made of two cysteine amino acids linked by a sulfur bond, found in proteins and important for the structure of hair, skin, and connective tissues.
From Greek 'kystis' (bladder) because it was first isolated from bladder stones in the 1800s; the name stuck even though we now know it's much more widely distributed in the body.
Your body makes cystine specifically to create extra-strong molecular bonds—it's like nature's spot-weld for proteins, and without enough cystine, your hair becomes weak and brittle.
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