Two main types of RET/PTC oncogene, named RET/PTC-1 and 3, occur in papillary thyroid carcinomas especially in those from Belarus children after the Chernobyl nuclear accident. Several variants of RET/PTC-3 have also been found, having different break points with respect to the classical RET/PTC-3. To our knowledge, no variant of RET/PTC-1 has been described up to now. We found a post-Chernobyl papillary thyroid carcinoma with an RET/PTC-1 rearrangement characterized by a transcript longer than expected. Sequence analysis of the PCR product obtained after RT-PCR revealed new fusion points between H4 and RET genes. The genomic sequence showed new breakpoints in both H4 intronic and in RET exonic regions. The RET gene breakpoint occurred within exon 11, at variance with the classical form of RET/PTC-1, in which it is in intron 11. As a consequence of this new fusion point, the transcript included 132 nucleotides of exon 11, coding for 44 amino acids of RET protein. Regarding the H4 gene, the classical breakpoint is in the first intron and the cDNA contains a fragment of 339 nucleotides. In our case the cDNA had a longer fragment of H4 involving a total of 1266 nucleotides. Sequencing of genomic DNA revealed a rearrangement breakpoint at position 886 of a new H4 intron located downstream of the 1266 coding region. Furthermore, as a consequence of the activation of a cryptic splicing site, 132 nucleotides of this intron were spliced between the H4 and RET genes. Sequence analysis of the new chimera showed that the original frames of H4 and RET were joint with the intronic sequence without disruption of the open reading frame (ORF). Moreover, the genomic DNA of this case showed transforming activity in the DNA-mediated transfection assay using NIH-3T3 cells. In conclusion, we describe here the first variant of RET/PTC-1 oncogene, which we have termed 'long'-PTC-1, characterized by new breakpoints of both genes involved in the rearrangement and having transforming activity. Similar to previously reported PTC-3 variants, long-PTC-1 has been found in a post-Chernobyl papillary thyroid carcinoma confirming that RET/PTC rearrangements other than the classical forms (RET/PTC-1 and -3) are specifically associated with radiation-induced papillary thyroid cancer.