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The Philadelphia-negative chronic myeloproliferative disorders feature autonomous myeloid hyperproliferation and hypersensitivity to a number of growth factors, which most recently have been shown to be explained by a guanine-to-thymidine mutation in the Janus tyrosine kinase (JAK2) gene, implicating that phenylalanine is substituted with valine in position 617 (V617F mutation). JAK2 is of particular importance to haematopoiesis, since JAK2 proteins are activated mainly by the haematopoietic growth factors. The JAK2 mutation is present in most patients with polycythaemia vera and about 50% of patients with essential thrombocytosis and idiopathic myelofibrosis. The identification of the JAK2 mutation is a major molecular breakthrough in the understanding of the pathobiology of these disorders, and it is a new molecular marker to be used in the future classification of the diseases as well as a simple and rapid diagnostic test. The mutated JAK2 tyrosine kinase is an obvious potential target for a small-molecule inhibitor of tyrosine kinase activity.

PRV-1 is a new molecular marker within the Ph-negative chronic myeloproliferative disorders. PRV-1 is a useful, highly sensitive and specific marker in the differentiation between polycythaemia vera (PV) and secondary erythrocytosis (ET), and seems to identify those PV patients presenting in the early phase of the disease with dominating thrombocytosis and thus a clinical phenotype of ET. These PRV-1 positive ET patients can be regarded as having "masked" PV or, more accurately, as having early PV. Moreover, PRV-1 positivity may be associated with a particular risk of thromboembolic complications. The biological role of PRV-1 and the significance of alterations in PRV-1 gene expression levels during treatment remain to be clarified.

The development of human cytogenetics has gained momentum during the past 20 years and many chromosome aberrations have been described to be specifically associated with more than 60 constitutional syndromes and more than 27,000 acquired neoplastic diseases. Chromosome analysis has in later years been further refined by the application of fluorescent in situ hybridisation technologies which enables the detection of genetic rearrangements, depending on the method employed down to single gene levels. Chromosome analysis is not only important in the diagnostic situation, but even more so in the prognostication of a wide range of diseases and especially with regard to malignant diseases in the follow-up to monitor treatment response. In addition, cytogenetics play an important role for unravelling the biology of neoplastic disease and the addition of relevant fluorescent in situ hybridisation and chip technologies will contribute with important information on pathogenetic mechanisms of both constitutional and acquired disease states.

A three-year-old boy was diagnosed with acute lymphoblastic leukemia. Two years later, while on maintenance chemotherapy, the patient was readmitted due to thrombocytopenia. The thrombocytopenia was caused by parvovirus B19 infection as evidenced by the finding of specific DNA in serum and bone marrow samples. Previous reports have highlighted the role of parvovirus B19 as a haematological pitfall and the present case shows that this virus has the potential to mimic a leukemic relapse.

Cytogenetic investigation in renal cell tumours may supplement morphological and immunohistological investigation. Cytogenetic aberrations may be an adjuvant in several situations: 1. For distinction between renal cell adenoma (RCA) and renal cell carcinoma (RCC). 2. For identification of specific genetic diseases with an increased incidence of RCC. 3. As support for a diagnosis of Wilms Tumour (WT) in children and for identification of genetic diseases with a higher incidence of WT. 4. As a possible aid in the distinction of RCA and RCC from rare kidney tumours.