The aberrant secretion of GIP in CFRD is accompanied by failure of islet β-cells to respond to GIP, but not to GLP-1 infusions [523], illustrating subtle differences in the insulin secretory pathways of the two incretins resulting from disturbances in the cystic fibrosis transmembrane conductance regulator (CFTR) on β-cells. This evidence concerns the gene GCG and cystic fibrosis-related diabetes.