A monoclonal antibody is an antibody cloned (see cloning) from a single antibody-producing cell, and therefore consisting of a single type of immunoglobulin. Monoclonal antibodies are produced by fusing antibody-forming lymphocytes from mouse spleen with mouse myeloma cells. The resulting hybrid cells multiply rapidly (like cancer cells) and produce the same antibody as their parent lymphocytes. In addition to their use in research, monoclonal antibodies are valuable diagnostic tools and have also been developed as pharmaceutical agents for treating a variety of conditions. For example they are used in the detection and treatment of cancer, as each one recognizes different proteins on the surface of both malignant and benign cells. They can be used as monotherapy or to deliver drugs or radioactive materials (e.g., yttrium-90) directly to tumor cells to kill them or prevent further growth.