... exhibit the same pheno-type as the heg mutant fish – that of the dilated heartcovered by a single layer of myocardium. The similar-ity of the phenotype in these nonsense alleles suggestedthat these ... to the endothelium, with a primary defect in the neuronalcell. Alternatively, the defect may lie primarily in endo-thelial cells, and the CNS selectivity of the diseasecould be a result of ... dilatedheart phenotype. The myocardium proliferates to anormal number of cells, but instead of building into concentric layers to form the walls of the heart, the myocardial cells form into a single layer,...