Among all the fast growing science and technology, the research of human genes, or biological engineering as people call it, is drawing more and more attention now. Sometimes it's a hot topic discussed by people. The greatest thing that gene technology can do is to cure serious diseases that doctors at present can almost do nothing with, such as cancer and heart disease. Every year, millions of people are murdered by these two killers. And to date, doctors have not found an effective way to cure them. But if the gene technology is applied, not only these two diseases can be cured completely, bringing happiness and more living days to the patients, but also the great amount of money people spend on curing their diseases can be saved, therefore it benefits the economy as well. In addition, human life span(寿命)can be prolonged. Gene technology can help people to give birth to more healthy and clever children. Some families, with the English imperial(皇帝的)family being a good example, have hereditary(遗传的)dis-eases. This means their children will for sure have the family disease, which is a great trouble for these families. In the past, doctors could do nothing about hereditary diseases. But gene technology can solve this problem perfectly. Scientists just need to find the wrong gene and correct it, and a healthy child will be born. Some people are worrying that the gene research can be used to manufacture(生产)human beings in large quantities. In the past few years, scientists have succeeded in cloning a sheep therefore these people predict that human babies would soon be cloned. But I believe cloned babies will not come out in large quantities, for most couples in the world can have babies in very normal way. Of course, the governments must take care to control gene technology. What does 'these two killers' in the second paragraph refer to?