IGFBP1 and neoplasm: While MMTV-Neu tumors had low levels of IGFBP-1 independent of MMP-9 status, IGFBP1-1 expression was increased in the MMP-9 knockout C3(1)-Tag model compared to the MMP-9 wild-type C3(1)-Tag model, suggesting that the tumor-promoting effect of MMP-9 is dependent on the status of its substrate, IGFBP-1.