Shown are representative time-matched (as indicated on the left) embryos that were incubated with BrdU for 15′ immediately before fixation and staining with antibodies against Cyclin B and BrdU, respectively. BrdU incorporation visualises DNA replication and Cyclin B degradation is a marker for mitosis (Figure 1A). Columns 1–2 show the Cyclin B and BrdU staining in whole mount embryos. Columns 3–5 are magnifications of an epidermal region with single channel detections of Cyclin B (CycB), BrdU and the merge of both channels (BrdU in green, Cyclin B in red). Magnifications are oriented with dorsal side up. We previously described experiments showing the stereotyped cell cycle progression in wild type embryos (Günesdogan et al., 2010). AEL: after egg laying, scale bars: 100 µm in columns 1–2, 10 µm in columns 3–5. 3.5–4 hr AEL: (A) dorsal epidermal cells degraded Cyclin B, completed M14 and entered S15 as shown by the incorporation of BrdU (arrowheads). (B) Ventral cells underwent M14 and entered S15 (arrowheads). Accordingly, all cells re-accumulated Cyclin B and incorporated BrdU. 4.5–5 hr AEL: (C) all epidermal cells undergo S15 and thus show Cyclin B staining and incorporation of BrdU. At this stage, dorsal epidermal cells of wild type embryos already entered M15/S16 (Figure 2C). 5.5–6 hr AEL: (D–F) dorsal epidermal cells stopped incorporating BrdU and completed S15 (arrowheads). 6.5–7 hr AEL: (G and H) dorsal epidermal cells degraded Cyclin B and entered M15 (arrowheads). Some cells already went through M15 and showed BrdU incorporation, indicating that they had entered S16 (arrowheads, G). The division pattern in wild type embryos is highly stereotyped and controlled by developmental genes. In wild type embryos, cells around the tracheal pits, which are structures in the posterior of each parasegment, enter M16 first (Figure 2D). In 2xHis-GU embryos, M15 took place at a developmental stage, when wild type embryos progressed through or completed M16 in the epidermis. Interestingly, the M15 division pattern in 2xHis-GU embryos (H) resembled the M16 division pattern in wild type embryos (Figure 2D), suggesting that the developmental program proceeds normally in 2xHis-GU embryos. 7.5–8 hr AEL: (I) after completion of M15, cells entered S16 and showed BrdU incorporation (arrowheads). (J and K) During and after germ band retraction, most epidermal cells did not incorporate BrdU, indicating that they aborted S16. Wild type embryos at this developmental stage stop to proliferate after M16 by entering a G1/0 phase (Knoblich et al., 1994). 2xHis-GU embryos apparently also enter a G1/0 phase, enforced by the ongoing developmental program.