(A) Three ALK-positive NSCLC patients with high (patients 3020, 3567, and 3026) ELISA O.D. values were analyzed by Western blot and showed strong reactivity to NPM-ALK, EML4-ALK, and recombinant full-length ALK, but not to the extracellular domain (ECD) of ALK which is not included in ALK fusions. Serum from one ALK-positive NSCLC patient (patient 1446, A) and two ALK-negative NSCLC patients (B) with low ELISA O.D. values did not detect any of the ALK proteins by Western blot. (C) Western blots using serum from two ALK-positive ALCL patients are shown as a positive control. (D) A monoclonal antibody raised against the ALK extracellular domain (ECD) [28] detects only full-length ALK and the ALK ECD, but not the fusion proteins NPM-ALK or EML4-ALK.