Proto-oncogenes, the normal equivalents to the transforming genes of mammalian tumorigenic retroviruses, are implicated in essential biological processes of normal human cells, like differentiation and proliferation. In vivo and in vitro studies clearly demonstrate that activation of proto-oncogenes with subsequent transformation in tumorigenic oncogenes plays an important role in induction and acceleration of the malignant disease, and also determines metastatic spread and development of resistance to chemotherapy in certain neoplasias. Investigation of these genes and analysis of their activation state should routinely be introduced in staging of hematological neoplasias, thus contributing to refinement of histopathological classification, clearer discrimination between reactive and neoplastic conditions and understanding of pathophysiological processes. Furthermore, impact on definition of high risk patients and novel therapeutic concepts based on a better definition of tumour biology may be expected from these studies. "Molecular cytology" combines detection of oncogenetic mRNA and the relevant oncoprotein on the single cell level by using "mRNA-in situ hybridization" and immuno-histochemistry. Application of this technique will allow to determine the mechanisms regulating the expression of oncogenes to define functional heterogeneity of tumour cell subsets and to clarify the relevance of minimal residual disease.