WB (peptide competition) analysis of HEK293 cells transiently transfected with plasmids expressing ERK5 kinase domain (ERK5kin) and constitutively activated MEK5D-D using GTX25686 ERK5 (phospho Thr218/Tyr220) antibody prior incubated with the non-phosphopeptide corresponding to the immunogen (Lane 3), a generic phosphothreonine-containing peptide (Lane 4), a generic phosphotyrosine-containing peptide (Lane 5), or the phosphopeptide immunogen (Lane 6 ). The data show that only the immunogen phosphopeptide blocks the signal, demonstrating the specificity of the antibody.