We then specifically tested the importance of myeloid‐cell‐secreted TNF‐α in vivo by performing transplantation of wild‐type bone marrow cells (BMCs) into TNF‐α‐null mice and found that myeloid‐cell‐derived TNF‐α contributed to sarcopenia and reduced muscle cell fusion, establishing the role of myeloid‐cell‐derived TNF‐α in influencing muscle aging. The gene discussed is TNF; the disease is sarcopenia.