 During the early 2000s the Zetas were instrumental in the Gulf Cartel's domination of the drug trade in much of Mexico.After the 2007 arrest and extradition of Osiel Cárdenas Guillén, the Zetas seized the opportunity to strike out on their own.Under the leadership of Heriberto Lazcano, the Zetas, numbering about 300, gradually set up their own independent drug, arms and human-trafficking networks.