The Paraprotein reference article from the English Wikipedia on 24-Jul-2004
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Paraprotein

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A paraprotein is an abnormal protein in the urine or blood, most often associated with multiple myeloma.

These are immunoglobulins or immunoglobulin light-chains that are produced by the clonal proliferation of plasma cells. Paraproteins form a narrow band, or 'spike' in protein electrophoresis as they are all the exact same protein.

Light-chains in the urine are called Bence-Jones proteins.