The current protocol consists of the initial immunoreactive trypsinogen (IRT) assay which is used to identify those infants with elevated levels of IRT which is a sensitive but not specific indicator for CF followed by a one or two stage mutation analysis of the CFTR gene if the IRT value is ≥99.5th centile; commonly referred to as the IRT /DNA protocol. The gene discussed is CFTR; the disease is cystic fibrosis.