MAP2K1 and fibrosarcoma: Through MSTN, various pathways—TGF-β-activated kinase 1/Mitogen-activated protein kinase kinase (TAK-1/MAPKK) for p38 mitogen-activated protein kinase (p38 MAPKs) or rat sarcoma virus/rapidly accelerated fibrosarcoma/Mitogen-activated protein kinase kinase-1 (Ras/Raf/MEK1) for extracellular signal‐regulated kinases1/2 (ERK1/2)—activate MAPKs.