Isolabelling is a radiation-induced phenomenon.

Human lymphocytes were incubated during two mitotic cycles in the presence of 5-bromodeoxyuridine and differentiation between chromatids was obtained with combined "Hoechst 33258" and azur-eosine staining. Analysis of non-irradiated cells revealed numerous sister chromatid exchanges (SCE) and no abnormalities of "harlequine" appearance of chromosomes. When, however, the cells were irradiated, an identical staining (IS, isostaining) of some chromosomes or chromosome segments were observed. Production of IS was accompanied by decrease of the frequency of SCE, the total frequency of SCE+IS remained, however, the same as in control. An antagonism between SCE and IS was established: the frequency of SCE decreased in the cells with multiple IS, and chromosomes with both SCE and IS were only rarely observed. Thus, IS is neither an artifact nor a physiologic event but a phenomenon induced by radiation. The reliable existence of IS is considered as an evidence for binemic structure of chromatid. It is suggested that some mechanism of lateral spread of genetic information is involved in the production of SCE. If delayed by radiation, the spread could be restricted only to a fraction of chromosome cross-section resulting in IS.