Identification of proteins coimmunoprecipitating with NPM/ALK by mass spectrometry. (A) Silver-stained SDS-PAGE gel of anti-ALK1 immunoprecipitates from NPM/ALK⁻ K562 cells (lane 1), lysis buffer alone (lane 2), and NPM/ALK⁺ SUDHL-1 cells (lane 3). Bands were excised from the gel, trypsin-digested, and analyzed by MALDI-TOF. Identified proteins are indicated by arrows. (B) MS/MS sequencing of peptides identifying PSF (GenBank accession no. P23246) and NPM/ALK (GenBank accession no. AAA58698) proteins.