Cartilage primordia appear early in tracheal development (~E10.5) when TRP63 +BC precursors are already present but still rather immature, without evidence of additional early markers of BC fate, such as KRT5 or KRT15 expression, or a recognizable BC layer in the developing tracheal epithelium (Yang et al., 2018; Hines et al., 2013). Here, KRT5 is linked to breast cancer.