Cytogenetic examination of the NOR activity in a proband with 13/13 translocation and in her relatives.

NOR activity in a proband with 13/13 translocation and in her relatives was examined by NOR silver impregnation and by determination of the association frequencies. In the proband, besides the fused chromosomes 13, also a chromosome 14 and a 15 showed no NOR staining. Therefore the possibility could be ruled out that the loss of NORs was compensated by the activation of inactive NORs. However, in the proband, one chromosome 22 seemed to be more intensively stained by silver nitrate than in her parents. As in the proband, the association frequency remained constant because of an increased association tendency of chromosomes 22. The possibility is discussed that the loss of NORs was compensated by a higher NOR activity of one chromosome 22.