Thymus twice as nice for mice

Researchers have long assumed that a mouse has only a single thymus. This organ, located directly over the heart, generates immune system components called T cells that protect the body from many pathogens. However, new research has turned up a second thymus, located in the neck.

While generating mutant mice for an unrelated study, Hans-Reimer Rodewald of the University of Ulm in Germany and his colleagues noticed that some of the altered animals had no thymus.