CFTR and cystic fibrosis: Given the limitations of IRT alone, the Wisconsin NBS program leveraged the discovery of the cystic fibrosis transmembrane conductance regulator (CFTR) gene, and modified its CF NBS algorithm in 1994, becoming the first US state to utilize a two-tier program that combined IRT measurements with DNA (deoxyribonucleic acid) testing for the most common CFTR genetic variant, F508del [7].