The sequence of the CLZ domain exhibits periodic heptad repeats (a-b-c-d-e-f-g) n in which the 'a' and 'd' positions are hydrophobic residues (Fig. 1b) 8 . This motif is characteristic of coiled-coil domains 31, 32 . There are two noncontiguous heptad repeat regions in each sequence; the first has three repeats and the second has two repeats. The two regions are out of register, with the 'a' position of the first region becoming the 'd' position of the second region.