HAND1 and atrial septal defect: Using a similar strategy for generation of Hand1 TG mice, we obtained Twist1-WT and -DD transgenic TG mice and found that while -WT TG mice had comparable heart to control mice, Twist1-DD TG mice displayed hypertrophy and some of them had atrial septal defect (ASD) and ventricular septal defect (VSD) (Figure 2B–D).