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Even if holder has Klutz or is under the effect of Embargo, the holder's Speed is reduced by 50%.
In addition to the EVs normally gained from defeating an opponent, the holder will also gain 4 EVs in Defense. Pokerus multiplies the final amount of EVs gained by two, so it includes this boost.
In HGSS, if a Pokemon is equipped with Power Belt and is in the daycare with another Pokemon, the offspring Pokemon will receive the Defense IV from the Pokemon equipped with it. If both parents are equipped with a Power Item, the game randomly chooses which IV to pass down.