The illness is believed to be caused by a defective gene.据信，这种疾病是由基因缺陷引起的。Doctors believe the condition is caused by a genetic defect.医生们认为这种情况是由基因缺陷引起的。According to one school of thought, the disease is caused by a genetic defect.有一种观点认为，这种疾病是由基因缺陷引起的。