Tbx1-GFP can partially rescue endogenous Tbx1Cre/-loss-of-function. Histological transverse sections through the heart at E14.5. (A) Tbx1Cre/- mice are functionally null and have PTA and VSD. Tbx1Cre/+;Tbx1-GFPflox/+ mutants are viable with normal heart histology. (B) Tbx1Cre/-;Tbx1-GFPflox/+ mutants exhibit complete rescue of outflow tract septation although half of the embryos have double outlet right ventricle (DORV). (C) Tbx1Cre/-;Tbx1-GFPflox/flox mutants all have ventricular septal defect (VSD), primarily with persistant truncus arteriosus (PTA) and one example of DORV.