Abstract
Sister chromatid exchanges [SCE] in human ring chromosome 4 are described after 2 rounds of replication in the presence of 5-bromodeoxyuridine [5-BrdU]. These were present in 90% of the lymphocyte metaphases of a patient of small stature, microcephaly, bilateral ptosis palpebrae, hypoplastic thumbs and male genetalia, and impaired mental development. SCE are apparnetly due to switches in DNA polarity.

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