The mitochondrial transcription factor A (Tfam) is a mitochondrial protein encoded in the nucleus. The gene for Tfam spans about 10 kb and consists of seven exons and six introns. In human and rat, exon 5 can splite alternatively resulting in two Tfam isoforms. In order to investigate the role of the delta 5Tfam isoform in human cells, we studied its stability in vitro, then we carried out overexpression experiments in H1299 human cell line in order to clarify the in vivo effect of this shorter isoform of Tfam. The data obtained by Real time-PCR demonstrate that the overexpression of delta 5Tfam causes an increase of mitochondrial transcription, so also this isoform as a role in the mitochondrial process.