Monoclonal gammopathy of undetermined significance (MGUS) is a condition where an abnormal protein, called monoclonal protein or M protein, is found in the blood. This protein is produced by a type of white blood cell known as a plasma cell. MGUS is usually discovered incidentally during blood tests and does not cause any symptoms. While MGUS itself is not cancerous, it can sometimes progress to more serious conditions, such as multiple myeloma or lymphoma. Regular monitoring by a healthcare provider is important to detect any changes in the condition over time.