Preferential S-phase pairing of the imprinted region on distal mouse chromosome 7.

Fluorescence in situ hybridization (FISH) has been used to visualize the spatial orientation of homologous chromosome regions in interphase nuclei of exponentially growing mouse fibroblasts. Simultaneous labeling of replicating DNA with the halogenated base analog 5-bromodeoxyuridine (BrdU) shows preferential somatic pairing of the imprinted region on distal mouse chromosome 7 during the S phase of the cell cycle. Trans-interactions between oppositely imprinted chromosome regions may be important for the maintenance of imprinting.