Cyclin B1–null embryos can be driven through up to three divisions by injecting wild-type Cyclin B1 mRNA. (A) Wild-type mouse Cyclin B1–Venus mRNA (mB1-Venus) was injected into both blastomeres at the two-cell stage. Representative frames of Video 8 are shown, beginning at the eight-cell stage; wild-type Cyclin B1–Venus (top) and transmitted light channels (bottom). Note the clearly visible nuclear import of Cyclin B1 before NEBD. Bar, 20 µm. (B) Additional markers such as PCNA were coinjected with wild-type or mutant Cyclin B1 mRNA in one blastomere at the two-cell stage. Representative frames of Video 9 are shown, from the eight-cell stage onward. The two uninjected blastomeres, left of the white dotted line, serve as an internal control and remain arrested whereas the right half of the embryo progresses up to the 16-/32-cell stage. Top row shows channel merge of single plane through the center of the embryo; middle row shows projection of the RFP-PCNA channel. Bottom row shows projection of mB1-Venus channel (bar, 20 µm). (C) Quantification of rescue of Cyclin B1^−/− embryos by injection of wild-type mouse Cyclin B1 mRNA; 78% of Cyclin B1^−/− embryos divide beyond the arrest stage (taken as the six-cell stage to include the rare cases in which one or two blastomeres divide beyond the four-cell stage; n = 51, 10 independent experiments; left). Variation in the ability of injected wt mRNA to rescue (red bars, n = 51) compared with nonrescued Cyclin B1^−/− embryos (blue bars, n = 83, 21 independent experiments; right); ∼25% of embryos respectively divided to the 8-cell stage, to the 16-cell stage, and up to the 32-cell stage.