Bone marrow...

Bone marrow is the soft, spongy tissue found inside a person's bones, including the hip and thigh bones. Bone marrow contains immature cells called stem cells. These cells can develop into any type of cell in the body.

Many people with leukemia depend on their bone marrow for survival. If a disease affects the bone marrow, it can't produce blood cells; a bone marrow transplant may be the best treatment option.