To determine if proliferation of GBM cells might be directly facilitated by pericytes, therefore enhancing tumor growth, we measured cell proliferation as cumulative population doublings in GBM cells interacting with pericytes in vitro. Interestingly, we found that GBC-PC did not proliferate at all, as opposed to control pericytes that proliferated, as expected, exponentially with the time of culture (Figure 4A). However, proliferation of GBM cells was not inhibited by the presence of pericytes (PCC-GB). PCC-GB seemed to reach even higher proliferation levels than control GBM cells, although this difference was not significant. In addition, we analyzed the cell survival of pericytes to confirm that the obtained data were due to inhibition of proliferation and not increased cell death. We found that pericytes survived after interaction with GBM cells during long time cultures (72 hours) as well as control pericytes (Figure 4B).