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Members of the Proud Boys are seemingly letting the world know they are all in on Sydney Sweeney ... and it's not because of her jeans -- it's about her genes.

The far-right extremist group appears to have vandalized a billboard along an L.A. area freeway, covering it with a banner that reads ... "Proud Boys love Sydney Sweeney. She has the best blue genes." ABC7

It's unclear when the banner was put up or who exactly is behind it ... but according to reports, some locals are stunned. One resident tells KABC it makes them feel "angry, scared, disappointed."

Of course, this all has to do with American Eagle's controversial "Sydney Sweeney Has Great Jeans" campaign which has taken a life of its own ... with critics arguing the campaign it's filled with implicit r

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