SP1 and cancer: PARP-1 mediated addition of PAR to Sp1 can now be viewed as one out of a few other post-translational modifications, such as glycosylation and phosphorylation, that the cell may use to modulate the positive regulatory influence of Sp1 and thereby contribute to prevent any given cell to progress toward anchorage-independent, unregulated cell proliferation, the hallmark of all cancer cells.