The purpose of this study was to search for STA gene defects in three families with clinically typical Emery-Dreifuss muscular dystrophy. Emery- Dreifuss is an X-linked muscular dystrophy with humeroperoneal weakness and life-threatening, but treatable, cardiac abnormalities in male patients and in female carriers. The defect is in the gene coding for emerin, a 254 amino acid protein of unknown function. Complementary and genomic DNA from T lymphocytes from the reported patients and their family members were amplified, cloned, and sequenced. A novel mutation, a 26 base-pair deletion in three brothers and a carrier mother, was detected in one family. A splicing mutation with one base pair insertion and a five base-pair deletion, which have been described previously, were found in the second and third families, respectively. The additional novel mutation detected and the findings of three different mutations in these three families support the idea of genetic heterogeneity of Emery-Dreifuss muscular dystrophy with different mutations in different families.