Fig. 7. Steps leading to Mec1-Ddc2 accumulation and checkpoint activation by a DSB. Shown is a summary of the events involved in Mec1-Ddc2 accumulation at a DSB in budding yeast. yKu and the MRX complex bind rapidly and recruit Tel1 (ATM). At this early stage, a low level of Mec1-Ddc2 can also be detected. The resection of the end by Rad24 and the 9-1-1 complex (Rad17, Mec3 and Ddc1) leads to the loading of RPA. The OB-fold domain 1 of RPA recruits Mec1-Ddc2 and this is antagonised by Rad51. Resection and Mec1-Ddc2 recruitment both appear to be aided by Rad24/9-1-1. Sufficient accumulation of Mec1-Ddc2 becomes visible as a `focus' and the downstream checkpoint activation of Rad53, stimulated by Rad9, occurs.