The deadline for cities to apply to become Amazon’s second headquarters (or HQ2) is Thursday. While proposals have come from all over the map, it is unlikely that any metro area with fewer than 4 million people will make the grade, according to Everest Group, a Dallas-based consulting and research firm.

Only cities of that size are likely to have the both the basic infrastructure and base of skilled employees to supply Amazon’s needs. Amazon amzn has said the new location could eventually employ 50,000 skilled people with average annual salaries of more than $100,000. These are not blue-collar jobs, and filling skilled jobs in these numbers is not easy, Eric Simonson, managing partner of Everest’s research group, told Fortune.

Given that, Everest says Atlanta, Boston, Chicago, Dallas, New York, and Washington D.C. will likely be Amazon’s top candidates.

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No other company’s expansion is anywhere near as massive as what Amazon proposes to do with HQ2 and that poses challenges, according to Everest’s research. Amazon rival Walmart wmt comes closest with its recently announced plan to add 14,000 to 17,000 employees near its existing Bentonville, Ark. headquarters. And Apple aapl is moving an estimated 12,000 people into its new circular headquarters in Cupertino, Calif. But Apple and Walmart are essentially adding to an existing local campus, not launching a full-blown equal headquarters.

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It would be difficult for cities like Pittsburgh, Tampa, Jacksonville, Orlando, Charlotte, Cincinnati, or Denver to scale up to 50,000 skilled workers over the next decade, according to Everest.

Lots of researchers are taking their own educated guesses as to who will make Amazon’s short list. Business Insider recounts Moody’s picks.