US v. Holguin, No. 19-50158 (10-13-22)(Nguyen w/Bea; partial concurrence/partial dissent by Berzon).  It would be “prudent” to test expert witnesses under Daubert, especially with such expertise as gangs and when the experts were law enforcement.   Such testing did not occur in this Mexican Mafia case when the “experts”  were just lay witnesses and officers. Yet the  9th affirmed  under plain error because of the harmlessness. Other issues include dual role of law enforcement  as experts and lay witnesses, adequacy of jury instructions about how to use such testimony, and gang evidence.

Berzon dissents on the harmlessness of introducing drug jargon expert testimony.

https://cdn.ca9.uscourts.gov/datastore/opinions/2022/10/13/19-50158.pdf