The US Patent and Trademark Office, which has been re-examining Apple's pinch-to-zoom patent (US No 7,844,915) presumably at Samsung's behest, said Wednesday that all 21of its claims are invalid on the basis of prior art, specifically for anticipation and obviousness.

It's a preliminary decision.

As the Wall Street Journal says, the patent - well, at least claim 8 - was a cornerstone of Apple's infringement case against Samsung, which filed the PTO's decision with the federal court in San Jose, California that heard Apple's infringement case this summer.

Apple, which has called the patent the commercially most valuable of the three multitouch software patents-in-suit, is expected to appeal.

The PTO's decision could affect the $1.05 billion award Apple got from a California jury.

Samsung claims the PTO's decision supports its request for a new trial, which was denied the other day.

Of course Samsung is looking to cut the jury verdict.

It has a workaround that Apple claims still infringes.

In October the PTO threw out Apple's so-called rubber-banding patent.

The two decisions cast a pall over two of the three software patents-in-suit.

FOSS Patent says "tentative rejections don't affect the enforceability of a patent claim."

See www.fosspatents.com/2012/12/tentatively-invalid-most-valuable.html.