Tumour formations arise as a consequence of alterations in the control of cell proliferation as well as with disorders in interactions between cells and their environment that result in invasion and metastasis. Recent advances in understanding the genetic basis of malignant diseases have been dominated by research in colorectal cancer. Genetic alterations of several proto-oncogenes and tumor-suppressor genes (e.g. APC/MCC, RAS, DCC, p53 mutations and/or allelic losses, hyperexpression of c-MYC and RB genes), as well as other genomic alterations, appear at characteristic stages of tumor development and are observed in most neoplasms. Generally, the normal cell has multiple independent mechanisms that regulate its growth and differentiation potential, and several separate events would, therefore, be needed to override these control mechanisms, as well as induce the other aspects of the transformed phenotype, like metastasis. These signals may be either positive or negative, and the acquisition of tumorigenicity results from genetic changes that affect these control points following a multistep mode. Statistics of the frequency of cancer incidence with age in humans indicate that for the genesis of e.g. lung carcinoma, five or six steps are required. Other types of cancers, such as leukemias and sarcomas, probably require quite a different number of rate-limiting changes. One of the best characterized tumours to provide a genetic model is colorectal tumorigenesis. Mutations implicated in breast cancer tumorigenicity are also studied and used as a genetic model in the literature worldwide. Finally, activation of c-abl in chronic myelogenous leukaemia (CML) and acute lymphoblastic leukaemia could also be presented as an example, which provides probably the strongest evidence for the role of proto-oncogenes in human malignancy process.