The chromosomal translocation t(1;6)(p35.3;p25.2), recurrent in chronic lymphocytic leukaemia, leads to RCC1::IRF4 fusion.

The chromosomal translocation t(1;6)(p35.3;p25.2) is a rare but recurrent aberration in chronic lymphocytic leukaemia (CLL). We report molecular characterization of 10 cases and show that this translocation juxtaposes interferon regulatory factor 4 (IRF4) on 6p25 with regulator of chromosome condensation 1 (RCC1) on 1p35. The breakpoints fell within the 5' untranslated regions of both genes, resulting in RCC1::IRF4 fusion transcripts without alterations of the protein-coding sequences. Levels of expression of both RCC1 and IRF4 proteins were not obviously deregulated. The cases showed other mutations typical of CLL and we confirm previously reported skewing towards the IGHV-unmutated subtype. RCC1::IRF4 fusion characterizes a rare subset of CLL.