MAPK3 and breast cancer: YKL-40 mediates activation of mitogen-activated protein kinases and phosphoinositide 3-kinase pathways through phosphorylation of extracellular signal-regulated kinase 1/2 and protein kinase B. Expression of YKL-40 is elevated in a variety of cancers, including breast cancer, colon cancer, lung cancer, prostate cancer, bladder cancer, and GC, and promotes tumor progression by promoting tumor cell proliferation, rapid invasion and migration, and angiogenesis [31].