Background: Basic helix loop helix (bHLH) proteins play a critical role in the differentiation of not only striated muscle cells but also adipocytes, neuron cells and smooth muscle cells, Previous studies have established in vitro mouse mesangial cells (MCs) to maintain the differentiated smooth muscle phenotype. Materials and methods: The purpose of the present study was to clone bHLH proteins from these MCs using the primers designed from a homologous sequence specific to bHLH, and to analyze the presence of bHLH proteins in normal kidney in vivo. From the cloning of MCs in vitro, we identified myf5 and herculin mRNA but not myoD. The expression of bHLH proteins in vivo was examined by immunohistochemistry with each specific antibody. Results: The MCs in newborn mice possessed Id but did not express either protein herculin or myoD. On the other hand, mature MCs expressed both myf5 and herculin. The Id protein disappeared in mature glomeruli. Conclusion: These results suggest that bHLH proteins are an important factor for mature MCs in vivo.