PSF coimmunoprecipitates with ALK fusion proteins expressed in cell lines and ALCL tumors. Reciprocal immunoprecipitation (IP) and Western blotting (WB) experiments were performed using anti-ALK1 and anti-PSF antibodies on lysates derived from (A) NPM/ALK⁺ SUDHL-1 cells and NPM/ALK⁻ K562 cells; (B) BaF3-NPM/ALK (BaF3-N/A), BaF3-Parental (BaF3-PAR), and BaF3 cells expressing kinase-dead NPM/ALK (BaF3-KD); (C) lymph node tissue taken from 2 patients with NPM/ALK⁺ ALCL and NPM/ALK⁻ spleen tissue; and (D) BaF3 cells expressing GCN4/ALK (BaF3-GCN4/ALK). As an internal control, SUDHL-1 and K562 cells were subjected to IP with an anti-IgG antibody (A).