Tumorogenesis is associated with several events by which a normal cell transforms itself into a tumour cell with an increased proliferation rate. One of the most important research initiatives in this area is the characterization of the molecular mechanisms involved in tumorogenesis and cancer. Oncogenes and tumour suppressor genes are directly involved in the cell cycle, differentiation, and apoptosis. The cellular oncogene MDM2 seems to be abnormally elevated in several human tumours, specially in sarcomas. The MDM2 gene product, mdm2 protein, pS3 and retinoblastoma (Rb) proteins, play crucial roles in the control of the cell cycle. The molecular interactions between mdm2, pS3 and Rb in cancer, are associated with a loss of control in the G1 phase of the cell cycle leading to uncontrolled cell proliferation. Studies by gene amplification appear to show an incomplete picture of mdm2 protein levels in tumour cells. The simultaneous determination of mdm2 protein and mRNA levels seems to give a more accurate interpretation of the abnormal function of the mdm2 protein. Thus, in addition to gene amplification, different mechanisms by which mdm2 is overexpressed in cancer cells also play an important role in tumorogenesis. (Rev Méd Chile 2000; 128: 539-46)