GPT and osteosarcoma: All of the glioma lines assessed retain expression of both ATRX and DAXX, as assessed by immunoblotting (Fig 1A) and immunohistochemistry (S1 Fig), while U2-OS, a well-characterized ALT-positive osteosarcoma cell line harboring an ATRX deletion [24, 43], lacks ATRX expression (Fig 1A).