jumonji (jmj) mutant mice, obtained by a gene trap strategy, showed several morphological abnormalities including neural tube and cardiac defects, and died in utero around embryonic day 11.5 (Et11.5). It is unknown what causes the embryonic lethality. Here, we demonstrate that exogenous expression of jmj gene in the heart of jmj mutant mice rescued the morphological phenotypes in the heart, and these embryos survived until E13.5. These results suggest that there are at least two lethal periods in jmj mutant mice, and that cardiac abnormalities may cause the earlier lethality. In addition, the rescue of the cardiac abnormalities by the jmj transgene provided solid evidence that the cardiac abnormalities resulted from mutation of the jmj gene. (C) 2004 Elsevier Inc. All rights reserved.