A stem cell capable of regenerating both bone and cartilage has been identified in bone marrow of mice. The cells, called osteochondroreticular (OCR) stem cells, were discovered by tracking a protein expressed by the cells. Using this marker, the researchers found that OCR cells self-renew and generate key bone and cartilage cells, including osteoblasts and chondrocytes. The discovery has implications for bone repair, the scientists say.