RAD1 is a component of a heterotrimeric cell cycle checkpoint complex, known as the 9-1-1 complex, that is activated to stop cell cycle progression in response to DNA damage or incomplete DNA replication. This complex also contains the Rad9 and Hus1 proteins and is believed to be involved in cellular responses to DNA damage, possibly by associating with Rad17 and several components of the PCNA-loading heteropentamer, replication factor C.