the biochemical defect in bloom 's syndrome ( bs ) remains unknown , but two characteristic features of bs cells point to a disturbance of dna replication , namely , an excessive number of sister-chromatid exchanges ( sces ) in bromodeoxyuridine ( brdu ) -substituted cells and an abnormally slow rate of replicon elongation .
C0037211
sister-chromatid exchanges
C0007634
cells