XRN2 and Abnormal cardiac septum morphology: Although neither Nkx2-5+/-nor Xrn2+/- hearts showed a muscular ventricular septal defect (VSD) at postnatal day 0 (P0), we found a VSD in Nkx2-5+/-Xrn2+/-newborn hearts (n = 3 of 7) (Figure 6).