A novel type of mutation – due to expansion of DNA trinucleotide repeats – has been discovered about 10 years ago. Nowadays 15 genetic syndromes and diseases caused by these mutations are known such as FRA X A syndrome, FRA X E syndrome, Kennedy syndrome spinobulbare muscle atrophy, Curschmann-Steinert syndrome of myotonic dystrophia, Huntington disease, Friedreich ataxia, spinocerebellare ataxias types I., II., III., VI., VII., VIII., XII. and Taylor’s oculopharyngeal muscle dystrophy. The mutations of instable trinucleotids represent some exceptions from the regular monogenic transmission such as premutation, genomic imprinting, generation anticipation (acceleration, accentuation), somatic mosaicism. A good understanding of their special properties is necessary for efficient interdisciplinar collaboration of medical teams taking care for these patients and their families.

        Key words: triplet repeats, premutation, fragile X syndrome A and E, Curschmann-Steinert syndrome, Huntington chorea, Friedreich ataxia, spinocerebellar ataxias, Taylor syndrome oculopharyngeal muscle dystrophy.