Transcription of the nar operon is activated under hypoxic/anoxic conditions and further stimulated by the presence of nitrate (25), so it is unsurprising that nar gene expression (26, 27) and the metabolic products of nitrate respiration have been detected in CF patient sputum (21, 28) and that Nar antibodies have been detected in the sera of CF patients (29). Here, CPSF4 is linked to cystic fibrosis.