Comparative RT-PCR analysis of cells from mild asthmatic donors versus non-asthmatic controls (severe asthma patients were excluded because of relatively poorer cell yields) during this period showed that the epithelial differentiation markers MUC5AC, PROM1 and CHI3L1 mRNA expression were significantly reduced after 30 days in culture when compared with expression at the time of sampling (day 0) and this reduction was similar whether cells were obtained from asthmatic or non-asthmatic donors (Fig. 2A). Here, CHI3L1 is linked to asthma.