Exogenous retinoic acid signalling results in the loss of early thryroid gene expression. When embryos are treated with retinoic acid (RA) at stage 24/26, expression of sftpb was detected in the thyroid (red arrows) and in the lung (green arrows) but it is only expressed in the lung in control (DMSO) embryos. Pax2 and foxe4 are normally expressed in the thyroid although both have distinct expression domains in other tissues. Embryos treated with RA still expressed both pax2 and foxe4 but the expression in the thyroid was eliminated. Expression of foxe1 could be observed in all embryos, particularly in the pharynx, but expression in the thyroid was undetectable. Treatment with the retinoic acid antagonist at stage 26 did not eliminate the expression of any tested markers in either the thyroid or lung.