Juvenile myelomonocytic leukemia (JMML) is an aggressive cancer of young children initiated by mutations in NRAS, KRAS and other genes encoding proteins that modulate Ras signal output. Hematopoietic stem cell transplantation (HSCT) is the only curative treatment and patients with relapsed and refractory disease have dismal outcomes. This trial evaluated the safety and efficacy of trametinib in advanced JMML. Kinase enrichment proteomic analysis was performed using available patient samples pre- or post-treatment with trametinib (NCT03190915) to evaluate the effects on the functional kinome.