Providers and suppliers use G modifiers on claims they expect to be denied as either not “reasonable and necessary” (GA and GZ modifiers) or because the items or services are not covered by Medicare (GY and GX modifiers). Such modifiers may be used when the provider is uncertain if a claim should be paid (for instance, when the provider does not know if a beneficiary already has had a test that is covered only one per year), or if the beneficiary needs Medicare to deny the claim so it can be submitted to the beneficiary’s secondary insurance. Medicare paid about $744 million for Part B claims with G modifiers in 2011. The OIG found vulnerabilities payments for such claims, since Medicare contractors often do not consider the modifiers to indicate that providers expect the services or items to be denied as not reasonable and necessary or not covered by Medicare. The OIG also reports that Medicare paid $4.1 million for claims with inappropriate combinations of G modifiers from 2002 to 2011. The OIG discusses ways CMS and its contractors could address the vulnerabilities presented in this report through automatic claims denials.