NOS1 and osteosarcoma: We measured TOP3A mRNA expression in these nine osteosarcoma cell lines by quantitative reverse transcription polymerase chain reaction (RT‐qPCR) and found that its expression was higher in the ALT‐positive ATRX‐wt cell lines than in the ALT‐positive ATRX‐mutated cell lines, except NOS1 (t = 2.642, df = 7; P = 0.033) (Fig EV5).