Abstract： Background Neuroblastoma (NB) is one of the most common malignant solid tumors of childhood.It is still not clear whether the apoptosis of tumor cells or the non-tumor cells contributes to the increase of concentration of cytochrome c (Cyt c) in the serum of the cancer patients.The aim of this research was to identify the source of the Cyt c in the serum when the tumor grows up by subcutaneous inoculation of human NB cells into nude mice.Methods We subcutaneously inoculated human NB cells (KP-N-NS) into nude mice and collected the sera of tumor-bearing mice (n=14) and control mice (n=25) 4 weeks later in order to screen for and identify differentially expressed proteins in the serum.Differentially expressed proteins in the serum were screened by surface-enhanced laser desorption/ionization-time-of-fiight (SELDI-TOF) mass spectrometry.Results The relative intensity of a protein having a mass-to-charge ratio (m/z) of 11 609 was 3338.37±3410.85 in the tumor group and 59.84±40.74 in the control group,indicating that the expression level of this protein in the tumor group was 55.8 times higher than that in the control group.Serum proteins were separated and purified by high-performance liquid chromatography (HPLC).Liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS) was performed to produce peptide mass fingerprints (PMFs).Spectrum analysis and a database search revealed that the highly expressed protein (m/z=11605.4) from the serum of tumor-bearing mice was the mouse Cyt c.Conclusions Increased concentration of Cyt c in the serum of tumor-bearing nude mice might be partially attributed to the secretion of this protein by non-tumor cells.