Important strategies against cancer are based on the understanding of the mechanisms of tumor progression. To elucidate alterations regarding tumor progression, we have performed proteomic differential display analysis for the expression of intracellular proteins in the regressive murine fibrosarcoma cell clone QR-32 and the progressive malignant tumor cell clone QRsP-11, derived from QR-32, by means of combination of two-dimensional gel electrophoresis (2-DE) and liquid chromatography-tandem mass spectrometry (LC-MS/MS), and we have previously reported on relevant results. However, besides the protein spots which we already reported, we identified three more particular spots of interest. In the present study, two-dimensional western blot analysis demonstrated a significantly lower expression of three isoforms of nucleophosmin in progressive, compared to regressive cell clones. These results suggest that the down-regulation of the identified nucleophosmin proteins in QRsP-11 cells compared to QR-32 cells is possibly related to tumor malignant progression.