[Rare case of mosaicism for chromosome 18, karyotype: 46, XX, del(18) (p11)/46, XX, i(18q)].

Rare mosaicism of chromosome No 18 is described. The proposita is 5.5 years old and has two cell clones: 50% of cells are monosomic for 18p and 50% have isochromosome i18q. The ratio of these clones (1:1) is found to be similar at the age of the proposita 2.5 and 5.5 years. The proposita has some phenotypic characters of both 18p- (ptosis, epicanthus, deformed carious teeth, falled back sternum etc.) and trisomy 18q (contraction of external auditory meatus, femur luxatus congenitus etc.) syndromes. A possible mechanism for the origin of such a mosaicism is discussed.