Editor's note: The formal investigation of Zhou Yongkang marks the fall of the most senior-level official since the reform and opening up of China in 1978.

Zhou is a former member of the Standing Committee of the Political Bureau of the Communist Party of China's Central Committee. There are currently 25 members in the Political Bureau, including seven members on the Standing Committee.

The members of the Standing Committee of the Political Bureau are top-nation level officials, and the other members of the Political Bureau are of vice-national levels. Zhou is the only national-level official under probe since 1978.

A total of four members of the Political Bureau have been investigated in the past 34 years.

Chen Xitong, former mayor of Beijing, was sentenced to 16 years imprisonment with the crimes of embezzlement and dereliction of duty on July 31, 1998. He was released on bail for medical service in May 2006. He died on June 2, 2013, at the age of 82.

Chen Liangyu, 68, former Party secretary of Shanghai, was sentenced 18 years in jail on April 11, 2008, with the crimes of accepting bribes and abusing power. The court also confiscated cash and private assets from him.

Bo Xilai, 65, former Party secretary of Chongqing, received life imprisonment on September 22, 2013, with the crimes of accepting bribes, embezzlement, and abuse of power. He appealed and the Shandong Higher People's Court maintained the verdict on Oct 25, 2013.

Xu Caihou, 71, former vice-chairman of China's Central Military Commission, was expelled from the Communist Party of China on June 30. He has been transferred to military prosecutors for further investigation on bribery charges.