49 CFR § 383.37 - Employer responsibilities.
No employer may allow, require, permit, or authorize a driver to operate a CMV in the United States if the employer knows or should reasonably know that any of the following circumstances exist:
(a) During any period in which the driver does not have a current CLP or CDL or does not have a CLP or CDL with the proper class or endorsements. An employer may not use a driver to operate a CMV who violates any restriction on the driver's CLP or CDL.
(b) During any period in which the driver has a CLP or CDL disqualified by a State, has lost the right to operate a CMV in a State, or has been disqualified from operating a CMV.
(c) During any period in which the driver has more than one CLP or CDL.
(d) During any period in which the driver, or the CMV the driver is operating, or the motor carrier operation, is subject to an out-of-service order.
(e) In violation of a Federal, State, or local law or regulation pertaining to railroad-highway grade crossings.