The human osteosarcoma cell lines OS99-1, Hu09, MG-63 and Saos-2 and the canine osteosarcoma cell lines D-17, UW0S-1, and UWOS-2 are all capable of forming spheres which express the embryonic stem cell genes Oct4 and Nanog and therefore have a primitive phenotype. This evidence concerns the gene POU5F1 and osteosarcoma.