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The results of a routine and fluorescence microscopic investigation of chromosomes are presented in 208 normal adults and 141 mentally retarded children (80 with diagnosis of "mental retardation of unknown etiology" and 61 with Down's syndrome). The occurence of both real chromosome aberrations and chromosome variants are presented, with special attention being paid to the chromosome variants. The data obtained show that in all groups studied there is considerable polymorphism of heterochromatic regions that appears to be dependent on sex and connected with the origin of some pathologic conditions.