The Mec1(sc)/Rad3(sp) protein family is central to the checkpoint pathways of cells. Functions upstream and downstream of Mec1(sc)/Rad3(sp) show both similarities and differences when compared between organisms. Analogy with a related protein, DNAPK(cs), suggests that different subunits may activate Mec1(sc)/Rad3(sp) in response to specific DNA or DNA-protein structures.