Temporal expression of lung and thyroid markers. Whole mount in situ hybridization was used to compare the temporal expression of key early genes in the development of the thyroid and lung. Expression of nkx2.1 is first detectable in the thyroid (red arrows) at stage 30 and is subsequently expressed in the lung (green arrows) at stage 34. Expression of pax2 begins at stage 32 in the thyroid. Expression of the differentiation markers sftpb and sftpc is first detectable at stage 38.