Sternum pain is pain or discomfort in the area of the chest that contains the sternum and the cartilage connecting it to the ribs. However, in most cases, sternum pain is unrelated to the heart and caused primarily due to problems with the sternum itself or the nearby cartilage. One important organ in the chest is the thymus, a small butterfly-shaped organ located between the heart and the sternum, or breastbone. This organ belongs to the immune system, and its job is to produce T cells, a type of white blood cell.