MAPK3 and osteosarcoma: In order to investigate if the lack of response to RTK, MEK and ERK inhibition can be explained by the level of expression of their molecular targets, we assessed the Human Protein Atlas database in order to analyze the expression of VEGFR1, VEGFR2, VEGFR3, EGFR, HERr2, MEK and ERK1/2 in bone tissue and U2OS osteosarcoma cells.