Are Siamese cats hairless

No they should remain the same.

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It depends what light they're in, as ALL cats are tabbys, but you just can't see their stripes most of the time. The easiest places to see the stripes are the paws and the tail. --- No, not all cats are tabbies. Cats that are not tabbies are called selfs and you cannot see tabby markings anywhere. Your Siamese is a tabby or lynx-point Siamese. Not all shows accept this, but most do. If the markings are only on the tail and not on the face or legs, it may be considered a fault in breed standards.