Because we were not able to detect an abnormally large number of dying pulmonary cells in the other two-thirds of the ku70−/− mice, we speculate that apoptotic cells are hard to detect in vivo, unlike in cell culture experiments, since dead cells can be cleared by macrophages and neighboring cells, especially if the ku70−/− mice were in the stages of developing lung diseases. This evidence concerns the gene XRCC6 and lung disorder.