The traditional view is that cGMP-dependent protein kinase-1 (PKG1; encoded by Prkg1) is the principal downstream signaling molecule mediating nitrergic responses, and Prkg1 is expressed in ICC in the small intestine (Salmhofer et al., 2001), as also confirmed by the present study. This evidence concerns the gene PRKG1 and intrahepatic cholangiocarcinoma.