However, in vitro experiments (5, 6) show that protamine zinc insulin promotes proliferation of breast cancer and bladder cancer cells by increasing phosphorylation of the insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF-1R), also by activating downstream phosphatidylinositol 3-kinase/mitogen-activated protein kinase (PI3K/MAPK) signaling pathways. Here, IGF1R is linked to breast cancer.