At 3 hours Rex-induced damage results in eggs containing varying numbers of foci of apparently overreplicated and fragmented chromatin. Lethal Rex-induced damage preferentially occurs in the rDNA.

rDNA crossovers are a small surviving minority of Rex-induced events. Cytology of embryos reveals obvious chromosome damage in some of the embryos after only three embryonic divisions, more than half of the embryos die.

Semi-dominant, maternal-effect locus which causes an early mitotic exchange between blocks of rDNA in 1 to 10% of the offspring of Rex-bearing females. Rex affects chromosomes with two separated, complete or partial ribosomal-DNA regions, such as YSX.YL or In(1)sc^S1Lsc^4R. Chromosomes having separated blocks of heterochromatin that do not both include ribosomal genes, such as In(1)w^m4, are insensitive. Affected chromosomes may pair in either spiral or hairpin configurations; the former results in loss of the intervening material (e.g., Ybb^Rex, Dp(1;f)sc^S1Lsc^4R, Dp(1;f)w^m51bLw^m4R), the latter in the inversion of that material (Dp(1;1)sc^S1Lsc^4R, Dp(1;1)w^m51bLw^m4R) (Robbins and Swanson, 1988). Exchanges usually occur early enough to yield whole-body recombinant genotypes, but a minority of events yield half-half mosaics. The exchange event generally results in partial deletion of rDNA (Robbins).