All three CRLF2‐r result in overexpression of CRLF2 mRNA and protein; however, alone they are insufficient to cause overt leukaemia.5, 7 Interestingly, studies identifying CRLF2‐d patients by mRNA and protein expression have shown that some patients do not harbor one of the three known genomic rearrangements.8 The cause of this overexpression is currently unknown. This evidence concerns the gene CRLF2 and leukemia.