Peptide Competition and Stimulation: Extracts prepared from CSF-1 stimulated (1-4, 6) or unstimulated (5) PAE cells transfected with a chimeric CSF-1/VEGFR2 receptor 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 ab5475 antibody for two hours at room temperature in a 3% BSA-TBST buffer, following prior incubation with: no peptide (1, 5, 6), the nonphosphopeptide corresponding to the immunogen (2), a generic phosphotyrosine containing peptide (3), or, the phosphopeptide immunogen (4). 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 ab5475 blocks the antibody signal, and the stimulation of the phospho signal after stimulating ligand is added (CSF-1 for the chimeric r