Sun Y, et al. – This paper highlighted the value of ancillary testing in the diagnosis of T–cell prolymphocytic leukaemia (T–PLL). Investigations suggested that TCL1 by immunohistochemistry (IHC) was a convenient test, positive in >80% T–PLL. However, conventional cytogenetics was found to be insensitive in the detection of 14q32/TCL1 rearrangements but provides complete information on the chromosomal landscape of T–PLL. In diagnostic challenging cases, fluorescence in situ hybridisation (FISH) for TCL1 rearrangement was very valuable.