The present invention is based on the identification and sequence determination of a novel gene, ALK, which is fused to the gene encoding nucleophosmin (NPM) in translocations present in t(2;5) lymphoma cells. Based on homologies to other proteins, the amino acid sequence of the polypeptide encoded by the ALK (Anaplastic Lymphoma Kinase) gene is a membrane-spanning protein tyrosine kinase (PTK)/receptor. Antibodies to the ALK PTK/receptor and methods utilizing such antibodies are described, as are methods of using the ALK gene to isolate ligands for the ALK PTK/receptor.