The proto-oncogene MYC is rearranged at its 3' end in the human T-cell leukemia line Hut 78 as a result of a translocation between the long arms of chromosomes 8 and 2. The nucleotide sequence at the breakpoint shows that the rearranged allele of MYC is truncated 24 nucleotides before the first poly(A)-addition signal. The 3' truncated MYC lacks a 61 nucleotide AT-rich sequence that has been reported to mediate selective mRNA degradation. We show that the truncation results in prolonged stability of MYC mRNA: the half life of the MYC mRNA in Hut 78, as well as in Rat 1A cells transfected with the truncated allele of MYC is increased by at least 5-fold. Our results document yet another mechanism by which MYC may be rendered pathogenic and dramatize the importance of mRNA stability in the regulation of MYC activity.