Orc1^KO cells are defective both in proliferation and amplification. however, these cells successfully complete developmentally programmed endocycles in salivary glands and reach a final ploidy of >1000C in the absence of zygotic Orc1 synthesis.

Both Orc1^KO homozygotes or Orc1^KO/Df(2R)ST1 transheterozygotes die soon after pupation. These mutants also demonstrate some developmental delays in the larval stages (molting and pupation occurs within a 1 day delay).

The hemispheres of Orc1^KO homozygous brains are significantly smaller compared with controls. BrdU incorporation is severely reduced in the entire Orc1^KO mutant brain, demonstrating a lack of DNA synthesis. No imaginal discs are detectable in late third instar larvae, indicating that this tissue, which normally increases exponentially in cell number during larval development, has failed to proliferate.

Orc1^KO mitotic clones do not show any changes in apoptosis but do not proliferate.

Orc1^KO follicle cell clones contain two to three cells, always fewer than their sister clones, consistent with the idea that Orc1 is required for proliferation.

Orc1^KO third instar salivary gland cells are large. The also incorporate BrdU in pulse-labeling experiments, showing that these cells continue to synthesise DNA late in larval development. The reduced size of Orc1^KO salivary glands seems to be due primarily to the presence of fewer cells than there are in the corresponding wild-type glands. Orc1^KO cells are the same size as wild-type cells.

The rate of DNA synthesis is essentially indistinguishable in Orc1^KO mutants from wild-type, even 100 hours into larval development, just before the onset of pupation. At the end of larval development, however, wild-type cells have approximately double the amount of DNA as Orc1^KO mutant cells. This DNA accumulates continuously throughout development in the wild-type nuclei, whereas accumulation is punctuated by two pauses that correspond to the larval molts in mutant nuclei.