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Nkx2.5 is a homeobox containg transcription factor that is conserved and expressed in organisms that form hearts. Fruit flies lacking the gene(tinman) fail to form a dorsal vessel, mice that are homozygous null for Nkx2.5 form small, deformed hearts, and several human cardiac defects have been linked to dominant mutations in the Nkx2.5 gene. If the levels of Nkx2.5 mRNA are experimentally increased during Xenopus heart development, hearts are enlarged. In addition, small, aberrant hearts are formed when Nkx2.5 protein is modified so that its DNA binding domain remains intact, but its activation domain is replaced with a repressor domain. Here we investigate the consequences of the overexpression of two truncated forms of Nkx2.5 that have been identified in humans with congenital heart defects, mRNAs encoding these mutations were injected into single cell Xenopus embryos, and their heart development was monitored. Our results indicate that the introduction of Nkx2.5 variants leads to aberrant heart formation, which is especially evident during inspection of the atria of the heart. This type of experiment may provide an approach to determining the functional consequences of expression of human genes linked to developmental defects.