Multiple myeloma is a type of cancer that targets the plasma cells in bone marrow. Healthy plasma cells attack and kill germs. A change in the bone marrow or a mutation in a plasma cell’s DNA can cause the cell to become malignant and reproduce rapidly. The malignant cells form a plasmacytoma. A person who has more than one plasmacytoma has multiple myeloma. Multiple myeloma tumors form in bone but can spread to other organs.