The MTCP1 gene is involved in the t(X;14)(q28;q11) translocation associated with T-cell prolymphocytic leukemia and related conditions. This gene is unusual in that it codes for two distinct proteins: a small mitochondrial protein, p8MTCP1, and a putative oncogenic protein, p13MTCP1. Scarcity of material from t(X;14)-associated proliferations and very low levels of mRNA expression have so far prevented a thorough description of p13MTCP1-encoding transcripts. Here, we characterize two additional t(X;14) bearing leukemias allowing this analysis. In one case, with a breakpoint located 5' to the MTCP1 gene, the level of transcription of previously described p13MTCP1-encoding transcripts is enhanced. In the second case, with a breakpoint within the MTCP1 intron I, an alternative transcription initiation site is demonstrated in the tumor cells at 229 bp upstream to exon II. The identification of this internal promoter, together with the similarity between TCL1 and MTCP1 genomic structures, allow us to propose a model in which the duplication of an ancestral gene was followed by the insertion of one copy within the intron of a p8-encoding gene, accounting for the unusual feature of the MTCP1 gene.