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Fig. 3 RA alters Lefty and Pitx2 gene expression. (A-C, ventral views) Treatment of mouse embryos for 4-6 hr with the retinoic antagonist AGN 193109 (10-7 M) down-regulates Pitx2 expression on the left LPM (arrow in A; wild type in B). Treatment of mouse embryos with RA (10-7 M) for 4-6 hr at the head fold stage induces Pitx2 expression on the right LPM (red arrow in C). (D-M) Treatment of zebrafish (D-F, dorsal views), Xenopus (G-J, lateral views), and chicken (K-M, dorsal views) with RA antagonist, or RA, down-regulates or induces Pitx2 expression, respectively (black arrows indicate normal expression in the left LPM and red arrows indicate ectopic expression in the right LPM). (N-P) Chicken embryos that were allowed to develop until stage 12 after treatment with RA antagonist (N) or RA (P) display a randomization of heart looping. Here we show hearts looping to the left (compare with the wild-type heart that loops to the right in O). (Q-V, ventral views) Treatment of mouse embryos at the head fold stage with RA antagonist (10-7 M) for 6 hr results in down-regulation of Lefty-1 and Lefty-2 genes in both the midline and LPM (arrows in Q; normal expression in R). On the other hand, similar treatment with RA induces ectopic expression of Lefty genes (red arrow in S). In the chicken embryo, when a bead soaked in RA antagonist is implanted on the left side of the node at stage 6, Lefty-1 expression is abolished (T; normal expression in U). On the contrary, RA beads implanted on the right side of the node of stage 6 chicken embryos induce Lefty-1 expression on the posterior right LPM (red arrow in V; black arrow indicates normal expression on the posterior left LPM). In the dorsal views (D-F and K-M), the left side of the embryo corresponds to the reader's left). WT, wild type.