 Affected horses have been found to trace to the stallion Poco Bueno, or possibly, farther back to one of his ancestors.Researchers have now named four deceased Quarter Horse stallions that were carriers and produced at least one affected HERDA foal; they are Dry Doc, Doc O'Lena, Great Pine, and Zippo Pine Bar.These stallions all trace to Poco Bueno through his son and daughter Poco Pine and Poco Lena.