The zinc knuckle is a zinc binding motif composed of the thefollowing CX2CX4HX4C where X can be any amino acid. The motifsare mostly from retroviral gag proteins (nucleocapsid).Prototype structure is from HIV. Also contains members involvedin eukaryotic gene regulation, such as C. elegans GLH-1.Structure is an 18-residue zinc finger. Zinc knuckle zf-CCHC