Giant panda "Mei Lun" receives medical checkup at the Atlanta Zoo in Atlanta, the United States, Nov. 12, 2013. Twin giant panda cubs "Mei Lun" and "Mei Huan" were 120 days old on Tuesday. They are the first surviving pair of giant panda twins ever born in the US. (Xinhua/Fang Zhe)

Giant panda "Mei Huan" waits for a medical checkup at the Atlanta Zoo in Atlanta, the United States, Nov. 12, 2013. Twin giant panda cubs "Mei Lun" and "Mei Huan" were 120 days old on Tuesday. They are the first surviving pair of giant panda twins ever born in the US. (Xinhua/Fang Zhe) 
 

Giant panda "Mei Lun" is taken by a veterinarian after its medical checkup at the Atlanta Zoo in Atlanta, the United States, Nov. 12, 2013. Twin giant panda cubs "Mei Lun" and "Mei Huan" were 120 days old on Tuesday. They are the first surviving pair of giant panda twins ever born in the US. (Xinhua/Fang Zhe)

Giant panda "Mei Huan" waits for a medical checkup at the Atlanta Zoo in Atlanta, the United States, Nov. 12, 2013. Twin giant panda cubs "Mei Lun" and "Mei Huan" were 120 days old on Tuesday. They are the first surviving pair of giant panda twins ever born in the US. (Xinhua/Fang Zhe)

Giant panda "Mei Huan" waits for a medical checkup at the Atlanta Zoo in Atlanta, the United States, Nov. 12, 2013. Twin giant panda cubs "Mei Lun" and "Mei Huan" were 120 days old on Tuesday. They are the first surviving pair of giant panda twins ever born in the US. (Xinhua/Fang Zhe)

Twin giant panda cubs "Mei Lun" and "Mei Huan" wait for medical checkup at the Atlanta Zoo in Atlanta, the United States, Nov. 12, 2013. Twin giant panda cubs "Mei Lun" and "Mei Huan" were 120 days old on Tuesday. They are the first surviving pair of giant panda twins ever born in the US. (Xinhua/Fang Zhe)

Twin giant panda cubs "Mei Lun" and "Mei Huan" play in an incubator at the Atlanta Zoo in Atlanta, the United States, Nov. 12, 2013. Twin giant panda cubs "Mei Lun" and "Mei Huan" were 120 days old on Tuesday. They are the first surviving pair of giant panda twins ever born in the US. (Xinhua/Fang Zhe)

Twin giant panda cubs "Mei Lun" and "Mei Huan" wait for medical checkup at the Atlanta Zoo in Atlanta, the United States, Nov. 12, 2013. Twin giant panda cubs "Mei Lun" and "Mei Huan" were 120 days old on Tuesday. They are the first surviving pair of giant panda twins ever born in the US. (Xinhua/Fang Zhe)

Giant panda "Mei Lun" receives a medical checkup at the Atlanta Zoo in Atlanta, the United States, Nov. 12, 2013. Twin giant panda cubs "Mei Lun" and "Mei Huan" were 120 days old on Tuesday. They are the first surviving pair of giant panda twins ever born in the US. (Xinhua/Fang Zhe)

Giant panda "Mei Huan" waits for a medical checkup at the Atlanta Zoo in Atlanta, the United States, Nov. 12, 2013. Twin giant panda cubs "Mei Lun" and "Mei Huan" were 120 days old on Tuesday. They are the first surviving pair of giant panda twins ever born in the US. (Xinhua/Fang Zhe)

Giant panda "Mei Lun" receives a medical checkup at the Atlanta Zoo in Atlanta, the United States, Nov. 12, 2013. Twin giant panda cubs "Mei Lun" and "Mei Huan" were 120 days old on Tuesday. They are the first surviving pair of giant panda twins ever born in the US. (Xinhua/Fang Zhe)

Giant panda "Mei Huan" waits for a medical checkup at the Atlanta Zoo in Atlanta, the United States, Nov. 12, 2013. Twin giant panda cubs "Mei Lun" and "Mei Huan" were 120 days old on Tuesday. They are the first surviving pair of giant panda twins ever born in the US. (Xinhua/Fang Zhe)