Monoclonal protein: An antibody made by myeloma cells that is synthesized uncontrollably, as opposed to being synthesized in response to a germ. M-proteins are found in excessive amounts in the blood or urine of patients with multiple myeloma. They have an abnormal amino acid sequence and protein structure owing to mutations in the genes responsible for immunoglobulin production in the myeloma cells that make them. Unlike normal antibodies, M-proteins are unable to fight infections. The identification of an M-protein is important for diagnosis, and the level of M-protein can help measure treatment effectiveness.