Finally, we validate this methylation mark in patient specimens and show that in an in vitro and in vivo setting it serves as a biomarker of MCL1 dependency/inhibitor response in additional pediatric cancer types, including atypical teratoid rhabdoid tumor (ATRT), ependymoma (EPD), and osteosarcoma (OS). This evidence concerns the gene MCL1 and atypical teratoid rhabdoid tumor.