This move effect may be in need of research. Reason:How long it confuses the holder in Stadium 2. Does the confused Pokémon replacement glitch occur in Stadium 2 You can discuss this on the talk page.
Confuses the holder (for a duration of 256 turns) and raises its Attack by 2 stages as soon as it enters battle. It is consumed upon use.
The Berserk Gene is consumed even if the holder is already confused or already has +6 Attack stages when it enters battle (due to Baton Pass). If a confused Pokémon is switched out and replaced by a Pokémon holding a Berserk Gene, the holder will be confused for the remaining duration of the first Pokémon's confusion, rather than for 256 turns.
In Detective Pikachu, the Berserk Gene is referenced several times as part of the drug known as R, which can momentarily make Pokémon more powerful, but also causes them to become berserk. Its effects derive from Mewtwo's cells and the Berserk Gene within them.
Spoilers end here.
In Pokémon Gold, Silver, and Crystal, wildMewtwo are programmed to always be holding a Berserk Gene, but since Mewtwo can't be found in the wild in those games, the item cannot be obtained that way. Even if it could be found in the wild, the item would be consumed immediately, so it still wouldn't be obtainable from Mewtwo.