TBXT and Patent foramen ovale: We also included a suggestive QTL (2.8<LOD<4.3) for FOW located on MMU9 (LOD = 3.43), since its peak covered the T-box transcription factor gene Tbx20, variants in the human orthologue of which are known to cause familial septal defects and severe PFO (Kirk et al., 2007).