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This is a debated question and the rules vary depending on if you play tournaments or cash-games.
In cash-games the general rule is that the hand is still alive even if the hole cards are exposed in the middle of a hand. One of the most cited set of poker rules available is "Robert's Rules of Poker" created by poker expert Bob Ciaffone. Rule 12 in the General Poker Rules chapter states that "A card that is flashed by a player will play." Similar rules are found in other poker rules collections.
But tournaments are a different story, here the general rule seem to be that exposed cards are considered dead.
It's important to keep in mind that there are no universal poker rules available and the rulings can vary between card rooms and tournaments. Always check the local rules before sitting down at the table.