A post-Chernobyl papillary thyroid cancer, displaying a novel ELE1/RET oncogenic rearrangement with an anomalous fusion transcript, was molecularly characterized. In spite of the presence of a normal breakpoint in exon 5 of the activating ELE1 gene, the sequence of the rearranged genomic DNA showed a previously unreported intra-exonic breakpoint in the RET protooncogene. As a consequence, a cDNA sequence 93 nucleotides larger than the regular one, and with the exon 5 of ELE1 joined to exon 11 instead of exon 12 of RET, is formed. To characterize the product of this new oncogenic ELE1/RET rearrangement, here designated as RET/PTC4, we performed an immunoprecipitation and Western blot analysis on cell extracts from NIH3T3 transfectants. The results showed the presence of two isoforms of the chimeric protein, displaying a constitutive tyrosine phosphorylation. As expected, the molecular weight of this protein was higher than that of RET/ PTC3 protein (p80 and p85, instead of p76 and p81). Previous reports, from our and other laboratories, showed that post-Chernobyl papillary thyroid carcinomas are characterized by a high frequency (about 60%) of RET oncogenic rearrangements (Fugazzola et al., 1995; Klugbauer et al., 1995; Ito et al., 1994). These events predominantly involve ELE1 activating sequence, thus producing RET/PTC3 oncogene (Fugazzola et al., 1995; Klugbauer et al., 1995). Hence, this elevated frequency of RET rearrangements could increase the probability of selecting unusual events as that here described. Alternatively, targeted radiation effects could be responsible for the atypical RET rearrangement producing RET/PTC4 oncogene.