FeNO is straightforward to measure, but when used as a single biomarker, it has been disappointing in titrating CS in patients with moderate asthma.5 Periostin is a secreted matricellular protein from a number of tissues including airway epithelial cells and is upregulated by T2 cytokines (IL-4 and IL-13) and is associated with airway eosinophilia in patients with severe asthma.1 An assay for serum periostin is being developed to help identify a periostin-high responder population for the anti-IL-13 monoclonal antibody lebrikizumab. The gene discussed is IL4; the disease is asthma.