Molecular cytogenetic characterization of two types of chromosome 9 variants.

Structural variations in the pericentromeric region of chromosome 9 are common. Molecular cytogenetic characterization of two types of rare variants, each studied in two families, are reported. For convenience, the variants are designated as types 2 and 3. Type 2 variants contain an additional G-positive band in the short arm near the centromere. Type 3 variants have heterochromatin both above and below the centromere in addition to having an additional G-positive band in the short arm immediately distal to the heterochromatin. Fluorescence in situ hybridization using alpha-satellite and D9Z5 beta-satellite probes revealed two clear hybridization signals in type 3 variants, one corresponding to the position of the centromere and the other to the position of the additional G-positive band. Results of type 2 variants are less conclusive, probably due to the close proximity of the additional band to the centromere, but are suggestive of additional hybridization in proximal 9p. Our finding suggests that the type 3 variant, and possibly the type 2 variant, may represent dicentric chromosomes that are functionally stable and phenotypically inconsequential.