The son of jailed Mexican drug lord Joaquín El Chapo Guzmán has pleaded not guilty to drug-trafficking and money-laundering charges in the US.
Ovidio Guzmán, 33, was extradited from Mexico to the United States on Friday.
His arrest in January turned into a bloody stand-off between the security forces and cartel members in which 30 people were killed.
His father, who was one of the founders of the Sinaloa drugs cartel, is serving a life sentence in Colorado.
Ovidio Guzmán, known as El Ratón (The Mouse) is facing five charges, including conspiring to import fentanyl to the US.
In a brief 15-minute hearing, in which he appeared dressed in an orange prison jumpsuit and spoke through a translator, Ovidio Guzmán pleaded not guilty.
The next hearing has been scheduled for November.
Fentanyl, a synthetic opioid which is 50 times more powerful than heroin, is now the leading cause of death for Americans aged 18 to 49, according to data analysed by the Washington Post.
While it is sometimes prescribed by doctors to treat extreme pain, illegally manufactured fentanyl has flooded into the US and has fuelled a deadly trade in the opioid.
Fentanyl overdoses have been described as "the single greatest challenge we face as a country" by US Homeland Security Secretary Alejandro Mayorkas.
Drug Enforcement Administration (DEA) agents say the Sinaloa drugs cartel is the source of much of the illicit fentanyl smuggled into the US.
Since the arrest of El Chapo Guzmán in 2016 and his subsequent extradition to the US, four of his sons - known as Los Chapitos (Little Chapos) - allegedly took on leading roles in the cartel.
According to DEA chief Anne Milgram, "the Chapitos pioneered the manufacture and trafficking of the deadliest drug our country has ever faced".
In May, the brothers released a joint statement through their lawyer denying any wrongdoing and saying that they had been used as scapegoats.
"We have never produced, manufactured or marketed fentanyl or any of its derivatives," they wrote.
Ovidio Guzmán's influence became evident in 2019 when he was briefly arrested by Mexican armed forces.
The violence unleashed by members of the Sinaloa cartel in the wake of his arrest was such that Mexican President Andrés Manuel López Obrador ordered his release to avoid more bloodshed.
He was rearrested in January in a huge security operation which saw him transported from Sinaloa to Mexico City in a helicopter out of fear that Sinaloa men could try to spring him from a prison van.