Component of the gypsy chromatin insulator complex which is required for the function of the gypsy chromatin insulator and other endogenous chromatin insulators. Chromatin insulators are regulatory elements which establish independent domains of transcriptional activity within eukaryotic genomes. Insulators have two defining properties; they can block the communication between an enhancer and a promoter when placed between them and can also buffer transgenes from position effect variegation (PEV). Insulators are proposed to structure the chromatin fiber into independent domains of differing transcriptional potential by promoting the formation of distinct chromatin loops. This chromatin looping may involve the formation of insulator bodies, where homotypic interactions between individual subunits of the insulator complex could promote the clustering of widely spaced insulators at the nuclear periphery. Within the gypsy insulator complex, this protein binds specifically to a region of the gypsy element located 3' of the 5' long terminal repeat (LTR), and may also mediate interaction with other endogenous insulators at sites distinct from those recognized by Cp190. Component of the gypsy chromatin insulator complex, composed of Cp190, mod(mdg4) and su(Hw) (PubMed:7664338, PubMed:11350941, PubMed:11416154, PubMed:15574329). The gypsy chromatin insulator complex interacts with Topors via mod(mdg4) and su(Hw) (PubMed:16209949). Upon ecdysone stimulation, interacts with Nup98 (PubMed:28366641). Colocalizes with other elements of the gypsy chromatin insulator complex at multiple sites on polytene chromosomes and at nuclear insulator bodies. Expressed in all stages of development.