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An instance of a B-group ring chromosome with a unique mosaic pattern occurring in a newborn with cri du chat syndrome is described in detail. The patient initially demonstrated mosaicism with 98% of cells having 45 chromosomes and one ring and 2% having 46 chromosomes and no ring. Reasons for this mosaic pattern are discussed. The ring chromosome seemed stable in size but decreased in frequency as the patient aged. Autoradiographic studies on this ring chromosome, as reported by Miller et al. (1966), did not clearly indicate whether it was one of the late or early replicating B chromosomes. Dermatoglyphic and blood grouping studies are presented.