Fucoidan treatment either in vivo (in DMBA-induced mammary cancer in rats, at 200 and 400 mg/kg, by oral route, for 16 weeks) or in vitro (in MDA-MB-231 cells, at 6.25–25 mg/mL) decreased the levels of p-PI3K, p-AKT, and p-GSK-3b (Ser9). This evidence concerns the gene PIK3CG and breast cancer.