Visceral heterotaxy is characterized by a variable group of congenital anomalies that include complex cardiac malformations and situs inversus or situs ambiguus. These anomalies result from the inability of the embryo to establish normal left-right asymmetry during development. Different modes of inheritance in familial heterotaxy suggest that several genes may be involved. A very large family with X-linked heterotaxy has been reported. The Applicant proposes to undertake a five year career development program focused on the isolation and characterization of the gene(s) responsible for X--linked visceral heterotaxy. As a Pathology Resident the Applicant acquired a strong interest in the molecular biology of normal and abnormal development. The Institute for Molecular Genetics at Baylor College of Medicine provides an environment of general scientific excellence, experience in training young investigators, and well-recognized contributions to our understanding of human genetics. The Sponsor has been particularly successful in isolating and characterizing disease genes on the X chromosome. The Applicant proposed the study of X-linked heterotaxy to the Sponsor and together they examined and collected blood on the majority of family members. In Phase I, initial mapping studies will be performed utilizing highly polymorphic microsatellite markers. Following initial mapping of the disease locus, we plan to identify new polymorphic markers in the region in order to further refine the genetic map. Overlap cloning will be performed using yeast artificial chromosomes (YACs), and a search for submicroscopic deletions will be performed. In Phase II, YAC subclones will be used for the identification of genes from the region. The expression pattern and sequence analysis of the genes will be examined in normal individuals and in patients with heterotaxy to determine their involvement in the pathogenesis of this disease. Further studies will include the characterization of the structure and function of the disease gene and its products. Overall, the investigators seek to increase our basic understanding of the molecular biology of cardiac malformations in humans and of the development of left-right asymmetry in all vertebrates.