Mouse embryonic fibroblasts (MEF) with point mutation in somatic cytochrome C gene were generated and characterized. It was shown that substitution of lysine for tryptophan in position 72 (K72W) decreased the proapoptotic functions of cytochrome C in response to staurosporin treatment without disrupting its respiratory functions. The presence of this mutation did not affect the pattern of cytochrome C gene expression or its localization inside the cell. These cell cultures therefore represent an interesting model for study apoptotic signaling and physiological functions of cytochrome C.