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San Francisco • Backers of a marijuana legalization initiative say they have collected enough signatures for the measure to qualify for the California ballot this November.

A coalition that includes former Facebook president Sean Parker and backed by Lt. Gov. Gavin Newsom and the nation's leading marijuana advocacy groups says it has collected 600,000 signatures, a lot more than the 365,000 needed, well ahead of the July 5 deadline.

Newsom, a Democratic candidate for governor in 2018, and other supporters of the ballot measure known as the Adult Use of Marijuana Act plan to launch the official campaign Wednesday in San Francisco.

The measure would allow possession of 1 ounce of marijuana and cultivation of six marijuana plants for adults 21 and older.

The initiative would also place a 15 percent tax on retail sales of the drug.