In support of this, we demonstrate that compared to age-matched normal controls (n = 8, average age 65.6 ± 3.8 years), HBECs from COPD donors (n = 6, average age 63.5 ± 3.5 years) have a reduced capacity to differentiate into club (SCGB1A1+), goblet (MUC5AC+) and ciliated (acetylated tubulin+) cells on ALI culture, with no difference in the number of BCs (KRT5+) (Figure 6A–D). Here, KRT5 is linked to chronic obstructive pulmonary disease.