Earlier, in mid-2016, five BMAL1-edited macaques were born, all of which exhibited obvious circadian rhythm problems, such as sleep disorders. They had more activity at night and slept less than normal. One of these five became the parent of all the five clones born last year.

"Gene profiling revealed abnormal expressions of most rhythmic genes, and up-regulation in the genes associated with sleep deprivation, depression and aging in the BMAL1-edited monkeys," Chang said.

More important, mental problems related to circadian rhythm disorders were observed in the monkeys, which was not found in mouse models, showing one advantage of monkeys over mice.

"The monkeys frequently wrapped their arms around their heads, avoided eye contact and cuddled in the corner－typical behaviors of anxiety－when humans approached them. Such symptoms are hard to detect in mice and are different from those of wild monkeys," Chang said.