Boeing has booked a big order for its planned 777X aircraft from the plane’s first Asian customer.

Hong Kong airline Cathay Pacific is buying 21 of the plane’s 777-9X version. The order is worth $7.9 billion at list price, but airlines get large discounts on big orders so the real price is likely around $4.4 billion, according to data from aircraft valuation firm Avitas.

The 777X will be an updated version of Boeing’s best-selling 777. That is Boeing’s second-biggest plane, used by airlines for very long international flights. For the new version, Boeing plans to replace its aluminum wings with wings made from a lightweight, high-tech plastic, and make other improvements that will make the plane more fuel-efficient.

Boeing has booked orders for 259 of the planes, worth a total of $95 billion at list prices.

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Boeing builds the current 777 in Everett, Wash., but it is still deciding where it will build the 777X.