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"You couldn't ask for a better pedigree and as she goes further she'll get better."
September is now a best-priced 8-1 favourite for next year's 1000 Guineas.
O'Brien, who struck 12 months ago in the Chesham with subsequent dual 2000 Guineas winner Churchill, said: "You'd be delighted with her. She's out of Peeping Fawn by Deep Impact, so she's bred absolutely in the purple.
"She has a massive engine. She's a great traveller and quickens really well.
"She was very impressive in Leopardstown and what's unusual about her is she's not a very big filly, but she thinks she's big - she has a big personality.
"Like those real good ones, she has a big presence about her.
"She could be anything really. I think she was the first two-year-old that we had that won first time out this year.
"It's very exciting."
September was a first Royal Ascot winner for Japanese sire Deep Impact.
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