FURIN and chronic obstructive pulmonary disease: When lung function is considered, a significantly higher expression of TMPRSS2, furin and neuropilin-1 are observed in COPD with the poorest lung function (<30% FEV1 % predicted) and, only furin demonstrated a significant inverse relationship with FEV1 % predicted (R = − 0.4880; p = 0.0072) (Supplementary Fig. 5i–p).