Gamma-ray irradiation introduces single and/or double strand breaks into the DNA 
molecule of the cells. In the case of mammalian cells, these breaks are being 
repaired in general during the first hr following exposure to ionizing 
radiation. The article reports on the results obtained from testing the ability 
of cultured lymphocytes from patients with Down's syndrome to repair 
radiation-induced DNA single-strand breaks. The ability to repair was deduced 
from the study of the DNA sedimentation profiles in alkaline sucrose gradients. 
It was found that lymphocytes from Down's syndrome patients are less efficient 
in repairing single-strand DNA breaks than are lymphocytes from normal 
individuals. This significantly increased fraction of unrepaired DNA strand 
breaks might be associated with the unusually high level of radiation-induced 
chromosome aberrations as compared with normals.