Many types of human leukemias and lymphomas are associated with specific chromosomal translocations that correlate with specific histologic and immunologic phenotypes (1 ). Anaplastic large-cell lymphoma (ALCL) of T-cell or null-cell lineage is a clinically aggressive non-Hogkins lymphoma of which a large subset of cases carry the t(2;5)(p23;q35) translocation. Molecular cloning of the breakpoints has revealed that this translocation fuses the nucleophosmin (NPM ) gene on chromosome 5q35 with a protein kinase gene (anaplastic lymphoma kinase [ALK ]) on chromosome 2p23. This results in the formation of a fusion protein with the N-terminal sequence derived from the NPM gene and the C-terminal cytoplasmic sequences from the ALK gene, including the consensus protein tyrosine kinase residues (2 ). Studies of the t(2;5) in ALCL using restriction fragment length analysis demonstrated clustering of the breakpoints. More detailed analysis revealed that the translocation occurs on variable locations in both chromosomes. These are located within the same introns of both genes in essentially all patients (3 ). Consequently, the translocation does not alter the reading frames of both derivative chromosomes, and the fusion mRNAs and proteins are identical in most translocations.