mRNA expression of differentiation and tight junction markers. Primary cells and cell lines were cultured at ALI over 21–28 days. RNA was extracted at days 7, 14, and 21 during ALI differentiation for each cell line or donor. Expression of MUC5AC (a), β-Tubulin IV (b), E-Cadherin (c), and ZO-1 (d) was measured. Data are normalised to the housekeeping gene HPRT1. Data are representative of two independent experiments. Error bars show standard deviation. Yellow, red, and blue bars represent expression at days 7, 14, and 21 post-ALI, respectively.