A so-called 'muscle' cancer is not really a muscle cancer. Oncologists have discovered the cell type that gives rise to rhabdomyosarcoma, the most prevalent soft tissue cancer in children. Previously, scientists thought the cancer arose from immature muscle cells, because the tumor resembled muscle under the microscope. However, the researchers discovered the cancer arises from immature progenitors that would normally develop into cells lining blood vessels. Source 4n.