The IRS has been sending signals since the summer that it will be enforcing the Affordable Care Act’s employer mandate.

Those signals have culminated in the IRS starting the process of sending letters to businesses with 50 or more full-time or full-time equivalent employees—referred to as Applicable Large Employers, or ALEs—on what they owe for failing to comply with the Affordable Care Act’s employer shared responsibility mandate for IRS filings related to the 2015 tax year.