To account for the possibility of immune cells and other cell types present in HPS mice that could alter the total cell count and contribute to lower percentages of proSP-C+ cells, we quantified the percentage of proSP-C+ cells per total epithelial cells as denoted by NKX2.1 staining (Supplemental Figure 2, A–D). Here, NKX2-1 is linked to Hermansky-Pudlak syndrome.