To determine if there is a gene variant of protein serine-threonine phosphatase with EF hand (PPEF-1) in T-cell lymphoblastic lymphoma SUP-T1 cell line, both in silico and in vitro approaches were conducted. In silico, a cDNA clone showing similar sequence to PPEF-1 was isolated from the SUP-T1 cDNA library and named PPEF-1V. The full-length of the PPEF-1V cDNA clone is a 2135bp containing a 1503bp open reading frame extending from 188bp to 1690bp, which corresponds to an encoded protein of 501 amino acid residues with a predicted molecular mass of 57.8 kDa. Alignment on both PPEF-1V and PPEF-1 sequences showed that PPEF-1V is a 350bp deletion in the nucleotide sequence of PPEF-1 from 128-477bp and a 152-amino-acid N-terminal deletion in the amino acid sequence of PPEF-1. In vitro, PPEF-1V transcript fragment was only highly expressed in T-cell lymphoblastic lymphoma cell line. In conclusion, the present patent showed that PPEF-1V could be a potential target for diagnosis or treatment of T-cell lymphoblastic lymphoma.