Lymphocytes from patients with Down Syndrome (DS) present an increased frequency of chromosomal aberrations after exposure to ionizing radiation in the G 0 and G 1 phases of the cell cycle. However, irradiated or bleomycin-treated (BLM) SD lymphocytes in the G 2 phase do not show a significant increase in the frequency of aberrations. BLM is a radiomimetic drug that has been used in cytogenetic experiments to verify the ability of individual DNA repair. The aim of this study was to analyze the chromosomal sensitivity of SD patients to BLM in the G 0 , G 1and S of the cell cycle. Lymphocyte cultures, with and without the drug, were made from 4 patients with DS and 4 normal individuals, all male and less than 10 years old. 100 mitotic metaphases of each type of culture were analyzed for each individual and chromosomal and / or chromatographic aberrations were recorded. The results show a higher frequency of dicentric chromosomes, acentrics, rings and double minutes in the SD cells in the G 0 and G 1 phases , but not in the S phase, in relation to the normal individuals' chart. Possibly, the phase of the cell cycle in which the cell is treated is the main factor involved in the chromosomal sensitivity of lymphocytes from patients with DS.
