CFTR and cystic fibrosis: As a secondary test, the IRT level is usually remeasured in the 2nd–4th weeks of life, DNA sequencing analysis is performed to assess the presence of CF-causing mutations in hotspot regions, and next-generation DNA sequencing analysis is applied in order to determine any possible mutation in the whole cystic fibrosis transmembrane regulator (CFTR; NM_000492) gene [5].