Cystic fibrosis (CF) is a disease based on mutations in the CF transmembrane conductance regulator (CFTR) gene. The codon sequence for part of the CFTR mRNA is:

5' -ACC-ATT-AAA-GAA-TTT-AAT-ATC-ATT-GGT-GTT-TCC- 3'

...where ACC codes for the amino acid threonine, which is the first amino acid encoded by the region of the gene shown. In some individuals with cystic fibrosis, the equivalent codon sequence is:

5' -ACC-ATT-AAA-GAA-AAT-ATC-ATT-GGT-GTT-TCC- 3'

What kind of mutation is it?