mus209^B1/mus209²⁷³⁵ mutants have a similar frequency of single-strand annealing repair (SSA) compared to controls in a P{wIw.FRT} hemizygous assay to study DNA double-stranded break repair when assayed at 32^oC or 38^oC.

Homozygotes are viable at 22^oC, but die as pupae at 29^oC. Homozygous females (raised at the permissive temperature) do not produce eggs. Homozygous ovaries are small and poorly developed. They are partitioned into ovarioles, but these are rudimentary and devoid of late stage egg chambers, containing only stem cells and early stage egg chambers. Homozygous females have a defect in DNA double strand break (DSB) repair. mus209^B1/mus209²⁷³⁵ females are partially fertile. Ovaries from these females develop further than either of their homozygous mutant counterparts, but still show developmental defects. These can include defects in follicle cell proliferation or migration and egg chambers that contain approximately double the normal number of nurse cells. Some egg chambers appear normal. The syncytial embryos derived from these females show a range of defects. 10-20% appear wild type. Others show varying degrees of abnormality in distribution of the nuclei. Embryos with highly irregular nuclei often appeared to be arrested in cell cycle progression, while in others aberrant nuclear division leads to nuclear "fallout" from the cortex. The overall level of recombination on chromosome 2 is reduced in mus209^B1/mus209²⁷³⁵ females compared to wild type, but only marginally so. The proportion of distal exchange events is reduced, while the proportion of centromere-proximal exchanges is actually increased.

Heterozygotes with mus209^B1 are lethal at 29^oC.