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 of microdeletions is high (1:4000 to 1:30,000) because their etiologic 
mechanism 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.