Carboxypeptidase A3 (CPA3) is a mast cell (MC) protease and there is a well-known switching of MCs expressing tryptase only (MCT) to MCs expressing tryptase, chymase and CPA3 (MCTC) with a dominant expansion of the latter in airways of asthma cases and more specifically in the context of severe asthma48. The gene discussed is CPA3; the disease is asthma.