MAPK3 and Ewing sarcoma: Molecular response biomarkers have been investigated retrospectively in a number of the clinical trials, including phosphorylated-ERK1/2 (p-ERK1/2) and pRPS6, which were associated with a better response to the combination of sorafenib and everolimus (33), a PDGFRA D846V mutation that was linked to long term efficacy of sorafenib monotherapy (32), and amplification of FGFR3, FGFR4 and FLT4 (VEGFR3) genes which was detected only in the pazopanib-responsive ES patient (30).