Mayors from the largest U.S. cities warned President-elect Donald Trump on Wednesday of the potential economic harm he might cause if he wipes out a program that allows young illegal immigrants to remain in the United States.

They warned in a letter that as much as $9.9 billion in tax revenue would be lost over four years and $433.4 billion in U.S. gross domestic product would be wiped out over 10 years if he cancels a policy aimed at protecting these people from deportation.

DACA, or The Deferred Action for Childhood Arrivals policy, was created through an executive order in 2012. It allows undocumented young people brought to the United States before the age of 16 to remain without fear of deportation as they pursue a higher education, work or engage in military service. The deferred action is subject to renewal every two years.