Greater than 90% of cases contain a clonal rearrangement of the T-cell receptor. Oncogenic potential is conferred by upregulation of a tyrosine kinase gene on chromosome 2. Several different translocations involving this gene have been identified in cases of this lymphoma. The most common is a chromosomal translocation involving the nucleophosmin gene on chromosome 5. The translocation may be identified by analysis of giemsa-banded metaphase spreads of tumor cells and is characterised by t(2;5)(p23;q35). The product of this fusion gene may be identified by immunohistochemistry for ALK. The nucleophosmin component associated with the commonest translocation results in nuclear positivity as well as cytoplasmic positivity. Positivity with the other translocations may be confined to the cytoplasm.