From bedside to bench and back again, in this article we aimed to examine (1) whether lymphopenia of naïve T-cells is also reflected in the peripheral blood of Atm-deficient mice, the mouse model of the human disease A-T, (2) to what extend HSCT does influence distribution of naïve (CD62Lhigh/CD44low) and memory (CD62Llow/CD44high) CD4+ and CD8+ T-cells in these mice and (3) compare the results to the immunological findings of the transplanted A-T patients by our group (25). The gene discussed is ATM; the disease is lymphopenia.