- Cells from an immunodeficient patient (46BR) with a defect in DNA ligation are hypomutable but hypersensitive to the induction of sister chromatid exchanges. Henderson, L.M., Arlett, C.F., Harcourt, S.A., Lehmann, A.R., Broughton, B.C. Proc. Natl. Acad. Sci. U.S.A. (1985)