In conclusion, this study demonstrates that fucoidan significantly induces apoptosis by activation of caspases, loss of MMP, Bax translocation to mitochondria from cytosol, and release of cytochrome c from mitochondria to cytosol via activation of p38 MAPK in human leukemia U937 cells, providing that important mechanistic insights related to p38 MAPK-mediated apoptotic cell death by fucoidan in U937 cells. Here, CYCS is linked to leukemia.