Immunofluorescent staining shows Runx1 expression during in situ fetal-lung EHT. Twelve μm sections of a fixed frozen postconception week-20 human fetal lung stained with the EHT marker CD41, the nuclear EHT marker Runx1, the endothelial marker VECAD, and the nuclear stain Hoechst. Cells that are Runx1+ and VECAD+ are highlighted with a white arrow, whereas cells that are Runx1+ and CD41+ are highlighted with a yellow arrow. The lumen of a large vessel lined with VECAD⁺ staining is highlighted with a “+.” Additional Runx1 staining can be found in supplemental Figure 9.