= Solution
Analyze the corresponding region from a parallel preparative <SDS-PAGE> gel, digest the recovered <proteins> and identify peptides by <liquid chromatography-mass spectrometry>. The lane and gel region should match the $35\,\mathrm{kDa}$ signal, with an adjacent-background or control-sample region processed in parallel. Because several <proteins> can comigrate, \b[validate the candidate against the actual phospho-band], for example by immunoprecipitating the candidate and testing Ras-PST25 reactivity, using a target-specific <Western blot>, or showing disappearance of the band after specific depletion. Phosphopeptide identification provides additional evidence; mass alone or detecting a common contaminant in the excised region is insufficient.
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