T-cell prolymphocytic leukemia (T-PLL), a rare form of mature T-cell leukemias, and ataxia telangiectasia clonal proliferation, a related condition occurring in patients suffering from ataxia telangiectasia, have been associated to translocations involving the 14q32.1 or Xq28 regions, where are located the TCL1 and MTCP1 putative oncogenes, respectively. The MTCP1 gene is involved in the t(X;14)(q28;q11) translocation associated with these T-cell proliferations. Alternative splicing generates type A and B transcripts that potentially encode two entirely distinct proteins; type A transcripts code for a small mitochondrial protein, p8MTCP1, and type B transcripts, containing an additional open reading frame, may code for 107 amino-acid protein, p13MTCP1. The recently cloned TCL1 gene, also involved in translocations and inversions associated with T-cell proliferations, codes for a 14-kD protein that displays significant homology with p13MTCP1. We have generated rabbit antisera against this putative p13MTCP1 protein and screened for expression of p13MTCP1 normal lymphoid tissues and 33 cases of immature and mature lymphoid T-cell proliferations using a sensitive Western blot assay. We also investigated the MTCP1 locus configuration by Southern blot analysis. The p13MTCP1 protein was detected in the three T-cell proliferations with MTCP1 rearrangements because of t(X;14) translocations, but neither in normal resting and activated lymphocytes nor in the other T-cell leukemias. Our data support the hypothesis that p13MTCP1 and p14TCL1 form a new protein family that plays a key role in the pathogenesis of T-PLL and related conditions.