ATRX and osteosarcoma: Consistent with this observation, we found that among the five ALT‐positive ATRX‐mutated tumors, two carried mutations in TP53 and KDM4A/JMJD2A (E23K and K463Q; that also demethylates H3K9 and H3K36), suggesting that simultaneous inactivation of TP53, ATRX, and KDM4A/JMJD2A may also drive ALT in pediatric osteosarcoma.