Cytogenetic analysis of 58 [human] cells originating in 7 mammary carcinomas indicated the existence of characteristic numerical and structural modifications, concerning the long arm of chromosome 1, chromosomes 4, 5, 7, 14, 15 and X, and the short arm of chromosome 16. Establishing the alterations of chromosomal ''markers'' could not only depend on selective phenomena relating to modifications of the genome but also on the preferential reassociations of heterochromatic segments.