The findings from this study might also lead to the hypothesis that, of the two MMPs, MMP9 is the predominant regulator of proBDNF proteolysis (Figure 6 and Figure 7) and resulting effect on cytotoxicity (Figure 8) and suggest that proteolytic cleavage of proBDNF by MMP9 represents a mechanism (Figure 9) by which the opposing actions of proBDNF and mBDNF may be regulated in the media of lung cancer cell lines. Here, MMP9 is linked to lung cancer.