Meet Retro, a cloned rhesus monkey born on July 16, 2020. He is now more than 3 years old and is “doing well and growing strong,” according to Falong Lu, a researcher at the Chinese Academy of Sciences, who published a study in the journal Nature Communications that describes how Retro came to be.

Retro is only the second species of primate (灵长类动物) that scientists have been able to clone successfully. The same team of researchers announced in 2018 that they had made two cloned cynomolgus monkeys, which are still alive today. “We have achieved the first live and healthy cloned rhesus monkey, which is a big step forward, although the efficiency is very low compared to fertilized embryos (受精胚胎),” said Lu. “Currently, we haven’t had the second live birth yet.”

The first animal to be cloned — Dolly the sheep — was created in 1996 using a technique called SCNT, where scientists essentially reconstruct an unfertilized egg by joining a somatic cell nucleus (体细胞核) with an egg in which the nucleus has been removed. The Chinese team improved the technique further to clone the rhesus monkey.

During hundreds of failed cloning attempts, they realized that, in the early cloned embryos, the outer part did not develop properly. To address this problem, they performed a process called inner cell mass transplantation, which involved putting cloned inner cells into a non-cloned embryo, and that allowed the clone to develop normally. “We think that there might be additional… abnormalities to be fixed. Strategies to further enhance the success rate of SCNT in primates remains … our main focus in the future,” Lu said.

The researchers said that being able to successfully clone monkeys might help accelerate biomedical research given that there are limitations on what scientists can learn from lab mice. Research on nonhuman primates, which are closer to humans, has been crucial for lifesaving medical advances, including the creation of medicines against Covid-19, according to a report by National Academies of Sciences, Engineering and Medicine.

【小题1】What can be learned about Retro?

A．It is the second live cloned rhesus monkey.	B．It represents a major scientific breakthrough.
C．It is a cloned monkey from a fertilized embryo.	D．It has outlived the cloned cynomolgus monkeys.

【小题2】What is Paragraph 3 mainly about?

A．The purpose.

B．The process.

C．The difficulty.

D．The method.

【小题3】How did scientists address the problem from failed attempts?

A．Give the clone space to grow.	B．Fix additional abnormalities.
C．Change a non-cloned embryo.	D．Help inner cells develop properly.

【小题4】What is the purpose of the text?

A．To present the recent progress in cloning.	B．To reveal challenges in cloning monkeys.
C．To call for research on nonhuman primates.	D．To stress the importance of medical advances.