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A staining technique that detects sister chromatid exchanges (SCEs) has been used to examine the response of chromosomes in cultured Chinese hamster cells to a wide variety of mutagens-carcinogens. The test gives a very sensitive and rapid method for detecting chromosome mutagenicity of chemical agents and provides a powerful new method for detecting environmental mutagens.

Information is presented which has been obtained from an exhaustive examination of 44 probands with a supernumerary marker chromosome (mar) and their families. The data include the derivation of the mar, frequency in various populations, inheritance and possible effect on fertility, congenital abnormality, and mental ability. The practical problems in assessing the risk of abnormality in a foetus discovered during prenatal diagnosis to be carrying a mar, are discussed.

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