For example, CF isolates were more likely to have mutated AlgU, a key regulator of mucoidy (58) (with mutations occurring predominantly at the interface between this sigma-H factor and its negative regulator protein, MucA), and PcnA, a putative nicotinamidase (with mutations found within the protein core or at sites of protein-protein interaction; Figure S13). This evidence concerns the gene PCNA and cystic fibrosis.