['To analyse spectra of chromosome aberrations induced in vitro by low LET radiation, in order to characterize radiation damage mechanisms quantitatively.', 'Multiplex fluorescence in situ hybridization (mFISH) allows the simultaneous identification of each homologous chromosome pair by its own colour. mFISH data, specifying number distributions for colour junctions in metaphases of human peripheral blood lymphocytes 72 hours after exposure in vitro to a 3 Gy gamma-ray dose, were combined with similar, previously published results. Monte Carlo computer implementations of radiobiological models for chromosome aberration production guided quantitative analyses, which took into account distribution of cells among different metaphases and lethal effects or preferential elimination of some aberrations at cell division.'] Does computer analysis of mFISH chromosome aberration data uncover an excess of very complicated metaphases?