CFTR and cystic fibrosis: Although there are examples of single genes that can result in a change of phenotype in both human genetics (cystic fibrosis and mutations in the cystic fibrosis transmembrane conductance regulator [CFTR] gene [1]) and microbial genetics (Mycobacterium tuberculosis rifampicin resistance and RNA polymerase [rpoB] mutations [2]), phenotypes are often more complex.