Western blotting with antibody ALK1. Antibody ALK1 recognizes a protein of 200 kD, corresponding to the normal ALK receptor, in the Rh30 rhabdomyosarcoma cell line (lane A), whereas only a smaller band of 80 kD, representing NPM-ALK, is detected in the t(2; 5)-positive Karpas 299 cell line (lane B). The antibody also detects the 200-kD ALK protein in cells from case 1 (lane C). The negative control (normal tonsil) is shown in lane D.