Homozygotes die at the 3rd larval instar/pupal boundary. BrdU incorporation in third larval instar brains is reduced in homozygotes compared to heterozygotes. Cells are arrested in a metaphase-like state with chromosome congression and spindle defects. There are two types of spindle defects based on severity; in 47% of cases (Class I), most chromosomes appear to congress at the metaphase as a highly condensed chromosome mass, the mitotic spindle is unfocused and spindle fibres are wound about the chromatid mass, and sometimes uneven spindle poles are seen. In 42% of cases (Class II), chromosomes do not congress at the metaphase plate and instead chromatids are seen throughout the spindle and at the spindle poles. In addition, the spindle appears to be diffuse but more focused than Class I. In some cases the spindles are monoastral. Orc2¹ homozygotes, Orc2¹/Df(3R)293γ7 and Orc2¹/Orc2^γ4 mutants show replication defects, abnormally condensed chromosomes and a metaphase-like arrest.

Examination of mutant larval neuroblasts reveals many severe mitotic defects. More than 90% of mitotic figures are abnormal. Most frequently, mitotic chromosomes contain regions of undercondensed chromatin connected by normally condensed chromatin connected by normally condensed or even highly condensed chromatin, the same undercondensed region often visibly affected on both sister chromatids. No normal anaphase figures are seen and their frequency is greatly reduced compared to wild-type. Chromosome breaks and rearrangements are also seen - including single and double sister chromatid breaks complex rearrangements, small supernumerary chromosomes containing sequences derived from chromosome X 2 or 3, however heterochromatic sequences are only very rarely undercondensed in these chromosomes. Mutant salivary glands are of normal size and polytene chromosomes appear to have normal banding and polyploidisation. After 24 hours of BrdU feeding a 13-22 fold decrease in the percentage of mutant cells that have entered S phase is seen. There is also a significant increase in the number of cells seen to be in G1 phase. In mutants the metaphase to anaphase ratio is 13-21 fold higher than in wild-type cells. The temporal control of replication is altered in mutant larvae. Unlike wild-type (where late replication is confined solely to centromeric heterochromatic) some areas of euchromatin are seen to replicate very late, even after heterochromatic domains. The late replicating areas of euchromatin exhibit the greatest problems in mitotic condensation.

Shows no dominant effect on telomeric Position Effect Variegation (PEV) in stocks carrying a variegating w^+mW.hs allele at the telomeres of the second and third chromosomes.

Polytenisation of chromosomes is unaffected in homozygous larvae.

Imaginal discs of homozygous larvae are missing or degenerate. Defects in the cell cycle: few or no dividing cells, affects chromosome condensation and arrest of the cell cycle at metaphase.