HIV-1 Tat PTD (T-PTD) has been shown to be a useful carrier for delivering transcription factors into living cells. Tissue specific transcription factors tagged with T-PTD sequences could penetrate into cells and induce tissue-specific differentiation. Meanwhile, many eukaryotic transcription factors containing basic helix-loop-helix (bHLH) motifs have been reported to facilitate protein transduction. In the present work, we developed a method to achieve efficient differentiation of primary cells using transcription factors tagged with T-FTD. MyoD and myogenin are myogenic transcription factors having bHLH domains, and we examined the effects of MyoD, T-PTD-fused MyoD (T-PTD-MyoD) and T-PTD-fused myogenin (T-PTD-MyoG) proteins on the differentiation of mouse primary cells. Although mouse primary cells were differentiated to be multinucleated cells (myogenic differentiation) by the addition of unmodified MyoD protein, about four times more differentiated cells were observed when we used modified MyoD, whose N-terminal region was tagged with T-PTD. These results indicated that MyoD protein combined with T-PTD could induce effective cell differentiation. (C) 2011, The Society for Biotechnology, Japan. All rights reserved.