Heptad repeat

The heptad repeat is an example of a structural motif that consists of a repeating pattern of seven amino acids:
 "a b c d e f g"
 H P P H C P C
where H represents hydrophobic residues, C represents, typically, charged residues, and P represents polar (and, therefore, hydrophilic) residues. The positions of the heptad repeat are commonly denoted by the lowercase letters "a" through "g".
These motifs are the basis for most coiled coils and, in particular, leucine zippers, which have predominantly leucine in the "d" position of the heptad repeat.