CXCL12 and Pulmonary artery atresia: In summary, while all Cxcl12 and Cxcr4 (not shown) constitutively null embryos had a membranous VSD at E15.5 we found no examples of CAT, IAA-B or retro-oesophageal RSA, nor did we see any examples of pulmonary atresia (part of the tetralogy of Fallot spectrum).