Peptide Competition: Extracts prepared from chick embryo fibroblasts expressing Pyk2 and plated on fibronectin were resolved by SDS-PAGE on a 10% polyacrylamide gel and transferred to nitrocellulose. Membranes were blocked with a 4% BSA-TBST buffer overnight at 4^oC, then were incubated with 0.25 µg/mL ab4805 antibody for two hours at room temperature in a 3% BSA-TBST buffer, following prior incubation with: no peptide (1), the phosphopeptide immunogen (2), the nonphosphopeptide corresponding to the immunogen (3), a phosphopeptide derived from the corresponding region of FAK (4) or, a generic phosphotyrosine containing peptide (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 ab4805 comepletely blocks the antibody signal, thereby demonstrating the specificity of the antibody. Peptide Compe