[Molecular cytogenetic study on the case with 14p+ marker chromosome]

A 23-year-old female was found carrying a submetacentric marker chromosome 14p+. The short arm of the marker chromosome showed a homogeneously stained region in both G-banded and C-banded preparations and was silver-positive in its distal part in AG-NOR preparation. Using a 3H-labelled 7.3 kb fragment of 18S and 28S rRNA genes as a probe and by chromosomal in situ hybridization, it was demonstrated that the autoradiographic silver grains to be located along entire p+ part of the marker chromosome. The short arm of the 14p+ had 5.3 times as many autoradiographic silver grains as that of any other normal acrocentric chromosome, but had no greater NOR activity than did the other acrocentric. The difference of these findings suggested that most of the amplified rRNA genes on the 14p+ chromosome were inactive, they might be inactivated by a process involving DNA methylation.