p12 represents the smallest, so far poorly characterized subunit of the mammalian DNA polymerase delta (pol delta) heterotetramer. Previously, to gain a molecular understanding of endothelial cell activation by fibroblast growth factor-2 (FGF2), we identified an upregulated transcript in FGF2-overexpressing murine aortic endothelial cells (FGF2-T-MAE cells) showing 89% identity with human p12. Here, we cloned the open reading frame of the murine p12 cDNA and confirmed the capacity of overexpressed or exogenously added FGF2 to upregulate p12 mRNA and protein in endothelial and NIH3T3 cells with no effect on the other pol delta subunits. p12 expression was instead unaffected by serum and different mitogens. Also, anti-p12 antibodies decorated FGF2-T-MAE cell nuclei and their chromosome outline during metaphase. Small interfering RNA-mediated knockdown of p12 caused a significant decrease in FGF2-driven proliferation rate of FGF2-T-MAE cells, in keeping with a modulatory role of p12 in pol delta activity. Immunoistochemistry of FGF2-embedded Matrigel plugs and FGF2-overexpressing tumor xenografts demonstrated a nuclear p12 staining of angiogenic CD31(+) endothelium. p12 immunoreactivity was also observed in the CD45(+)/CD11b(+) inflammatory infiltrate. Thus, FGF2 upregulates p12 expression in endothelial cells in vitro and in vivo. p12 expression in infiltrating inflammatory cells may suggest additional, cell proliferation-unrelated functions for this pol delta subunit.