IL13 and asthma: The IL‐13 response genes, CLCA1 (chloride channel, calcium‐activated family member (a), POSTN (periostin) and SERPINB2 (serine peptidase inhibitor clade B member (b) have been shown to be upregulated in central airway bronchial epithelial cells in non‐steroid treated asthma 11 and normalized in the central airways by high‐dose inhaled steroid therapy in those with severe asthma.12