Peptide Competition: Extracts prepared from NIH3T3 cells left unstimulated (1) and stimulated with PDGF (2-5) were resolved by SDS-PAGE on a 10% polyacrylamide gel and transferred to PVDF. Membranes were blocked with a 5% BSA-TBST buffer overnight at 4°C, then were incubated with 0.50 µg/mL ab5443 antibody for two hours at room temperature in a 1% BSA-TBST buffer, following prior incubation with: no peptide (1, 2), the non-phosphopeptide corresponding to the immunogen (3), a generic phosphotyrosine containing peptide (4), or, the phosphopeptide immunogen (5). After washing, membranes were incubated with goat F(ab’)2 anti-rabbit IgG alkaline phosphatase and bands were detected using the Tropix WesternStar method. The data show that only the peptide corresponding to ab5443 blocks the antibody signal, thereby demonstrating the specificity of the antibody.