Bone marrow is a biological tissue found within the larger bones of many species. Generally, bone marrow produces oxygen-carrying blood cells (erythrocytes), among other types, or cells that can become fat, cartilage, or bone.
In 2371, Kes discovered the genetic markers in Seska's blood did not match that of a Bajoran, but rather appeared to be Cardassian. When confronted with this evidence by Chakotay, Seska claimed to have survived an outbreak of Orkett's disease due to a bone marrow transplant from a sympathetic Cardassian woman named Kattell. However, The Doctor knew that a bone marrow transplant could not explain the genetic markers, and Seska was exposed as a Cardassian spy. (VOY: "State of Flux")
In 2372, Vidiians who were invading a duplicate of Voyager extracted Kes' pelvic ridge in order to analyze her bone marrow for resistance to the phage. (VOY: "Deadlock")