Unique mosaicism of structural chromosomal rearrangement: is chromosome 18 preferentially involved?

The mentally normal mother of a 4-year-old boy with del(18)(q21.3) syndrome was tested cytogenetically to study the possibility of an inherited structural rearrangement of chromosome 18. She was found to carry an unusual mosaicism involving chromosomes 18 and 21. Two unbalanced cell lines were seen as derivatives of a reciprocal translocation t(18;21), resulting in mosaicism of two cell lines, one with partial monosomy 18q and one with partial trisomy 18q. A literature review revealed that mosaicism of two or more cell lines with different unbalanced structural aberrations is extremely rare; moreover, chromosome 18 appeared to be involved in the majority of cases. We discuss possible mechanisms for the origin of this distinctive chromosomal constitution. Copyright 2003 Wiley-Liss, Inc.