This protein motif is a zinc binding motif [1]. It contains 9 conserved cysteines that coordinate three zinc ions. It is thought that this region plays a structural role in stabilising SET domains.

1: Structure of the SET domain histone lysine methyltransferase Clr4. Min J, Zhang X, Cheng X, Grewal SI, Xu RM; Nat Struct Biol 2002;0:0-0. PMID:12389037