FGF signaling within the Isl1Cre lineage critical for outflow tract (OFT) morphogenesis. Deletion of the sequences that code for the `c' isoforms of FGFR1 and FGFR2 in double heterozygote mice demonstrate grossly (a,b) normal OFT rotation, septation, and orientation. However, Isl1Cre-conditional Fgfr1c and 2c isoform mutants demonstrate double outlet right ventricle (DORV) (c,d) and PTA (e,f). Histological analysis confirmed that OFT develop normally in Fgfr1c/Fgfr2c double heterozygote mice (b), but that combined conditional ablation of Fgfr1c and Fgfr2c isoforms results in OFT defects including DORV (d) and type-I persistent truncus arteriosus (PTA) (f). Ao, aorta; co, conus; PA, pulmonary artery; RV, right ventricle; TA, truncus arteriosus. (Reprinted with permission from Ref 181. Copyright 2008 The Company of Biologists Ltd)