Fig. 3. Clearance of maternally administered RA in rescued Raldh2^–/– embryos carrying RARE-lacZ. The time course of RARE-lacZ induction was examined in Raldh2^–/– and wild-type embryos following RA administration at 12-hour intervals from E6.75-E8.25. ß-galactosidase activity is shown at E8.5, 6 hours after the last RA dose (A,F); E8.75, 12 hours after the last RA dose (B,G); and E9.25, 24 hours after the last RA dose (C,H). For comparison, RARE-lacZ expression in untreated embryos of the same stages can be seen in Fig. 1. Expression of Cyp26A1 during RA clearance was monitored in Raldh2^–/– and wild-type embryos by whole-mount in situ hybridization at E8.75, 12 hours after the last RA dose (D,I); and E9.25, 24 hours after the last RA dose (E,J).