Retinoic acid is required for lung differentiation but excess retinoic acid causes expression of lung differentiation markers in the thyroid. When embryos were treated with a retinoic acid antagonist (RAA) at stage 14 of development, the lung failed to differentiate as judged by the lack of sftpb expression at stage 38/40. If the retinoic acid antagonist is not added until stage 26, the expression of sftpb in the lung was essentially normal. If embryos were exposed to exogenous retinoic acid (RA) starting between stages 20 to 34, expression of sftpb was detectable in both the lung (green arrows) and in the thyroid (red arrow). Addition of retinoic acid at earlier (stage 14) or later stages (stage 36) did not result in expression of sftpb in the presumptive thyroid. Carrier control (DMSO) embryos had normal expression of sftpb. Treatment times are indicated at the top of each column (eg. T-st14 indicates that the treatment was initiated at embryonic stage 14).