Although this is in qualitative agreement with the WB results the pYY/YpY intensity ratio is markedly lower in the MS/MS compared to the WB analysis (Fig. ). This difference likely originates from a limited antibody specificity. We therefore compared the level of pYpY after the two stimulations and as expected, the pYpY signal increased in intensity after the anti-CD3 + H2O2 stimulation (Fig. ). Crossreactivity of anti-p492 with pYpY could therefore account for the quantitative discrepancy between the MS/MS and WB analytical results.