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What is Most common primary malignant bone tumour is: and explain in detail?
Multiple myeloma is the most common primary neoplasm of the skeletal system. 
The disease is a malignancy of plasma cells. Plasma cells are a subset of B cells, which are the producers of humoral immunity factors termed antibodies. However, if malignant transformation occurs in a single plasma cell, its clones produce only a single type of immunoglobulin, and electrophoresis demonstrates a monoclonal peak corresponding to this particular immunoglobulin. 
Multiple myeloma has been the prototype of monoclonal malignancies, in this case, of plasma cells.
Reference: Shafer’s Textbook of ORAL PATHOLOGY Eighth Edition page no 241