New high-resolution cytogenetical technique identified an increased number of terminal, interstitial and subtelomeric microdeletion as the etiology of many syndromes of multiple congenital anomalies, mental retardation and facial dysmorphy. A loss of contiguous genes shows a high phenotypical variability and at the same time it is significant for genetic prognosis. 1) Variability of clinical features depends on the size and pathogenetic mechanism of underlying deletion; 2) Dysmorphic face features are of a characteristic type and can be somatoscopically recognized; 3) Heart defects and mental retardation are common features of microdeletion syndromes; 4) New mutations represent the most common etiology of microdeletions; only 1 to 10% of mutations are transmitted from the parental gonadal mosaics, from the balanced translocation or from the same microdeletion in parents; 5) Recurrence risk is low, but it may be as high as 50% in individual cases of inherited mutation; 6) Genetic heterogeneity is high and the responsible genes can be located at different chromosomes (e.g. Di George syndrome due to mutation on 22q or 10q) and can also result from microdeletion or point mutation (in the Shprintzen syndrome 70 % represent microdeletion and 30 % point mutation at 22q11, in Rubinstein-Taybi syndrome 10 % cases result from microdeletions and 90% from point mutations); 7) Population incidence ofmicrodeletions is high (1:4000 to 1:30 000) because their etiologicmechanism is related to the common unequal crossing over; 8) Imprinting plays a role in some cases, e.g. Prader-Willi syndrome results from nullisomy of paternal 15q12 chromosome, Angelman syndrome is related to that of maternal 15q12 chromosome; 9) Prenatal prevention of the high risk familial chromosomal rearrangements is feasible since the 12th gestation week.

        Key words: microdeletion, imprinting, contiguous genes, microdeletion syndromes, genetic prevention.