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Approximately 30% of myeloma patients express cyclin D1 RNA and protein. The low incidence of translocation t(11; 14) detected by conventional cytogenetics suggests that the up-regulation of cyclin D1 protein might result from other mechanisms as well as from gene amplification. Therefore, the frequency and the clinical and prognostic implications of cyclin D1 amplification were examined. We highly purified myeloma cells from bone marrow by magnetic cell sorting and analysed 50 myelomas by fluorescence in situ hybridization (FISH) using probes specific for cyclin D1 and 20 samples by immunoblotting to detect cyclin D1 expression. The amplification of cyclin D1 gene was found in 19 of 50 analysed patients and was associated with expression of cyclin D1 protein. The amplification correlated significantly with the bone marrow infiltration, plasma cell morphology and labelling index as well as serum β2-microglobulin, C-reactive protein (CRP) and creatinine levels. In univariate analysis, the amplification of the cyclin D1 gene was a significantly unfavourable parameter with regard to overall survival (P = 0·0064) and progression-free survival (P = 0·0005). In multivariate analysis, cyclin D1 amplification and serum β2-microglobulin were independent and well-suited parameters for predicting survival. The detection of cyclin D1 amplification seems to be of promising prognostic value in multiple myeloma. [ABSTRACT FROM AUTHOR]