<i>Furin</i> mutant hearts present a defect of the AV junction.

(A, D) Sections of an adult mouse wild type (n = 3) and mutant (n = 3) hearts (11 weeks old) stained with Masson-trichrome protocol to reveal connective tissue in blue. C and D show higher magnification images of the areas within the yellow squares in A and B. Note the myocardial continuity between the cardiomyocytes of the atria and the ventricle in mutants (yellow arrow in D). MV, mitral valve. (E-F) Sections of E18.5 mouse wild type (n = 2) and mutant (n = 2) hearts stained with Van-Gieson protocol to reveal connective tissue in red. G and H show higher magnification images of the areas within the black squares in E and F.