The Nkx2.5 gene encodes a transcription factor that plays a critical role in heart development. In humans, heterozygous mutations in NKX2.5 result in congenital heart defects (CHDs). However, the molecular mechanisms by which these mutations cause the disease remain unknown. NKX2.5-R142C is a mutation that was reported to be associated with atrial septal defect (ASD) and atrioventricular (AV) block in 13-patients from one family. The R142C mutation is located within both the DNA-binding domain and the nuclear localization sequence of NKX2.5 protein. The pathogenesis of CHDs in humans with R142C point mutation is not well understood. To examine the functional deficit associated with this mutation in vivo, we generated and characterized a knock-in mouse that harbours the human mutation R142C. Systematic structural and functional examination of the embryonic, newborn, and adult mice revealed that the homozygous embryos Nkx2.5(R141C/R14IC) are developmentally arrested around E10.5 with delayed heart morphogenesis and downregulation of Nkx2.5 target genes, Anf, Mlc2v, Actc1 and Cx40. Histological examination of Nkx(2.5R141c/+) newborn hearts showed that 36% displayed ASD, with at least 80% Of adult heterozygotes displaying a septal defect. Moreover, heterozygous Nkx2.5(R141C/+) newborn mice have downregulation of ion channel genes with 11/12 adult mice manifesting a prolonged PR interval that is indicative of 1st degree AV block. Collectively, the present study demonstrates that mice with the R141C point mutation in the Nkx2.5 allele phenocopies humans with the NKX2.5 R142C point mutation. (C) 2017 Elsevier Ltd. All rights reserved.