HeLa cells expressing either empty vector (control) or a Flag-RGS14 construct were treated with either with (A) PDGF-AB, 20 ng/ml; (B), EGF, 100 ng/ml; or (C) fetal calf serum, 20 % final. Cells were treated for 10 min and compared to unstimulated cells as control. Protein extracts (100 ug) were prepared and subjected to immunoblot analysis using anti-phosphoErk p42/44 antibody (Erk1/2), anti-Flag (for RGS14) and anti-β-COP (loading control). Erk1/2 phosphorylation results from three to five separate experiments for each condition were quantitated by densitometry analysis and the results pooled and averaged (+/− SEM), as described in Experimental Procedures. Immunoblot results are shown from a single representative experiment used for densitometry. Differences in control and stimulated conditions were compared and subjected to statistical analysis (paired t-test). RGS14 inhibited PDGF-stimulated Erk1/2 phosphorylation significantly (*P<0.01).