Reduced NOR association frequency in a 13/18 translocation chromosome. A family study.

The association frequency of a functional, intercalary NOR in fibroblasts of five carriers of a translocation t(13;18) was significantly lower than the average association frequency of normal G- and D-group chromosomes in the same carriers. This decreased coefficient of association does not seem to be attributable to partial loss of the NOR during rearrangement or to a loss of NOR activity, as indicated by silver staining. It is hypothesized that either (1) interphase associations including this chromosome form less frequently or, more likely, (2) associations with this chromosome are differentially susceptible to physical disruption due to flattening of association complexes during cell preparation. The presence of this abnormal NOR chromosome did not affect the association behavior of normal G- and D-group chromosomes in carrier cells. In addition, the present study supports previous observations that G-group chromosomes associate with a greater average frequency than D-group chromosomes.