Molecular characterization of the smallest secondary constriction region (qh) of human chromosome 16.

We report the smallest secondary constriction region (h) in human chromosome 16. The cytochemical, cytogenetic, and molecular techniques revealed the complex heterogeneity of heterochromatin observed in this region. The heteromorphisms can be found due to the variation in centromeric (c) region alone or in combination with the h region. Routine selective staining techniques fail to differentiate the C region from the h region. However, the fluorescence in situ hybridization technique clearly demonstrated that the centromere of chromosome 16, which is composed of 340-base-pair dimers arranged in a tandem array of 1.7-kb higher-order repeat units, is not heteromorphic in the present case, but other molecular cytogenetic techniques demonstrated the presence of a very small h region. The evolution of heterochromatin of this region is discussed.