Mario Alberto Cárdenas Guillén is a former leader of the Mexican criminal group called the Gulf Cartel.
 He is the brother of Osiel Cárdenas Guillén and Antonio Cárdenas Guillén.
Joaquín Archivaldo Guzmán Loera (] ; born on 25 December 1954 or 4 April 1957) is a Mexican drug lord who headed the Sinaloa Cartel, a criminal organization named after the Mexican Pacific coast state of Sinaloa where it was formed.
 Known as "El Chapo" ("Shorty", ] ) for his 168 cm stature, he became Mexico's top drug kingpin in 2003 after the arrest of his rival Osiel Cárdenas Guillén of the Gulf Cartel, and was considered the "most powerful drug trafficker in the world" by the United States Department of the Treasury.
Carlos Alberto Rosales Mendoza (12 February 1963 – 27 December 2015) was a former Mexican drug lord who founded and led an organized crime syndicate called La Familia Michoacana.
 He was a close friend and associate of Osiel Cárdenas Guillén, the former leader of the Gulf Cartel.
Homero Enrique Cárdenas Guillén (13 March 1966 – allegedly died on 28 March 2014), also known by his aliases El Majadero and El Orejón, was a Mexican suspected drug lord and alleged leader of the Gulf Cartel, a drug trafficking organization.
 He is the brother of the former Gulf Cartel leaders Antonio, Mario, and Osiel Cárdenas Guillen.
 During the late 1990s, Homero worked for the Gulf Cartel under the tutelage of his brothers.
 However, after several years of government crackdowns, the Gulf Cartel suffered severe drawbacks, including the death and arrests of Homero's brothers and allies.
 In August 2013, Homero became the de facto leader of the Gulf Cartel following the arrest of Mario Ramírez Treviño.
 However, he reportedly died of a heart attack on 28 March 2014.
Rafael Cárdenas Vela (a.k.a. "El Junior") is a former Mexican drug lord and high-ranking lieutenant of the Gulf Cartel.
 He is the nephew of Antonio and Osiel Cárdenas Guillén, two men who at one time led the criminal organization.
Osiel Cárdenas Guillén (born May 18, 1967) is a former Mexican drug lord and the former leader of the Gulf Cartel (Spanish: "Cártel del Golfo" ) and Los Zetas.
 Originally a mechanic in Matamoros, Tamaulipas, where he was born, he entered the Gulf Cartel by helping Juan García Abrego, the capo at the time; when García Ábrego was arrested in 1996, some infighting erupted within the cartel.
 Osiel Cárdenas eventually took control by killing his friend and contender Salvador Gómez, earning Cárdenas the nickname ""El Mata Amigos"" (The Friend-Killer).
Héctor Manuel Sauceda Gamboa (died 17 February 2009), commonly referred to by his alias El Karis, was an alleged drug trafficker and high-ranking leader of the Gulf Cartel (Spanish: "Cártel del Golfo"), a Mexican drug trafficking organization.
 He was the brother of the drug lord Gregorio Sauceda Gamboa, another high-ranking drug trafficker who worked under the tutelage of Osiel Cárdenas Guillén, the former top leader of the cartel.
Samuel Flores Borrego (a.k.a. Metro 3) (6 August 1972 – 2 September 2011) was a Mexican drug lord and high-ranking lieutenant of the Gulf Cartel.
 He was a former state judicial policeman who protected the ex-leader of the Gulf cartel, Osiel Cárdenas Guillén.
 Upon his arrest, Flores Borrego became the right-hand man of Jorge Eduardo Costilla Sánchez, the former leader of the criminal organization.
Efraín Teodoro Torres (died 3 March 2007) was a Mexican suspected drug lord and one of the founders of Los Zetas, a criminal organization formed by former soldiers of the Mexican Armed Forces.
 He joined the Mexican Army in mid-1991 but deserted after seven years of service.
 In 1998, Torres was recruited by the drug lord Osiel Cárdenas Guillén to join the ranks of the Gulf Cartel, a drug trafficking organization.
 The new group formed by ex-commandos came to be known as Los Zetas.
Los Metros is a faction of a Mexican drug trafficking organization known as the Gulf Cartel.
 The group was formed in the late 1990s during the reign of Osiel Cárdenas Guillén, the former leader of the cartel, to provide security to the organization's leaders as the cartel's armed wing.
 The leader of Los Metros, Mario Ramirez Treviño, alias X-20, was arrested on 17 August 2013.

Question: Mario Cárdenas Guillén is the brother of the Mexican drug lord who was the former leader of what group?
Answer: Gulf Cartel