RecG stabilizes the gene cluster instability of BLM- cells. Southern blotting detection of the rDNA clusters with resolution up to 1 Mb is shown. Cluster lengths are calibrated by S. cerevisiae chromosome size markers. Open star is the 1 Mb resolution limit of the gel: all clusters larger than 1 Mb co-localize to this band. Clonal populations are shown on the left of each panel, with a collection of subclones derived from this clonal population to the right. Red carets: mitotic recombination indicated by minor intensity gene cluster bands. The zone of particularly high instability in the 250 kb to 550 kb size range is indicated by red shading and by the red dotted bracket at the right. 6A) “BLM-: RecG 2a high”: BLM- cells stably expressing higher levels of cotranslated RecG/EGFP. 6B) The parental BLM- line from which the “BLM-: RecG 2a high” cells were derived.