An alignment of sequence of yeast iso-1-cytochrome-c (y-cyt-c) with that of the horse cyt-c suggests that the former has five extra N-terminal residues (www.uniport.org).We have been interested in understanding the role of this extra N-terminal extension in folding and stability of the yeast protein.For this we have prepared five deletants of y-cytc by sequentially deleting N-terminal residues, and did experiments to know the role of each residue in folding and stability of the wild type protein.Here we present our circular dichroism, absorption, fluorescence, dynamic light scattering and differential scanning calorimetric measurements on only one deletant in which all residues of the 5 N-terminal extension have been removed.The following main conclusions have been reached from our measurements.The N-terminal extension does not (i) affect the global secondary and tertiary structure, (ii) perturb local tryptophan-iron distance, (iii) influence Met80-heme and Phe82-heme interactions, and (iv) affect the heme spin state of the protein.It has also been observed that the N-terminal extension does not affect the optical and hydrodynamic properties of the native, heat-denatured, guanidinium chloride-(GdmCl-) denatured, pre-molten globule and molten globule states of the protein.However, the heat-denatured state contains significant amount of secondary structure that can be removed by GdmCl.It has been observed that the N-terminal extension does contribute to the thermo, thermal and chemical stabilities of y-cyt-c.For instance, on the removal of all 5 residues of the N-terminal extension, Tm (midpoint of thermal denaturation) is reduced by 4 摄氏度, and both Gibbs free energy changes associated with thermal denaturation and chemical denaturation are reduced by 18％.