BOSTON (AP) — As marijuana shops sprout in states that have legalized the drug, they face a critical stumbling block — lack of access to the kind of routine banking services other businesses take for granted.

U.S. Sen. Elizabeth Warren, a Massachusetts Democrat, is leading an effort to make sure vendors working with legal marijuana businesses, from chemists who test marijuana for harmful substances to firms that provide security, don’t have their banking services taken away.

It’s part of a wider effort by Warren and others to bring the burgeoning $7 billion marijuana industry in from a fiscal limbo she said forces many shops to rely solely on cash, making them tempting targets for criminals.

After voters in Warren’s home state approved a November ballot question to legalize the adult use of cannabis, she joined nine other senators in sending a letter to a key federal regulator, the Financial Crimes Enforcement Network, calling on it to issue additional guidance to help banks provide services to marijuana shop vendors.

Twenty-eight states have legalized marijuana for medicinal or recreational use.

Warren, a member of the Senate Banking Committee, said there are benefits to letting marijuana-based businesses move away from a cash-only model.